I'm building a web application with Django. The reasons I chose Django were:
Now that I'm getting closer to thinking about publishing my work, I start being concerned about scale. The only information I found about the scaling capabilities of Django is provided by the Django team (I'm not saying anything to disregard them, but this is clearly not objective information...).
"What are the largest sites built on Django today?"
There isn't any single place that collects information about traffic on Django built sites, so I'll have to take a stab at it using data from various locations. First, we have a list of Django sites on the front page of the main Django project page  and then a list of Django built sites at djangosites.org . Going through the lists and picking some that I know have decent traffic we see:
pownce.com  (no longer active): alexa  rank about 65k. Mike Malone of Pownce, in his EuroDjangoCon presentation on Scaling Django Web Apps  says "hundreds of hits per second". This is a very good presentation on how to scale Django, and makes some good points including (current) shortcomings in Django scalability.
HP had a site built with Django 1.5: ePrint center . However, as for novemer/2015 the entire website was migrated and this link is just a redirect. This website was a world-wide service attending subscription to Instant Ink and related services HP offered (*).
"Can Django deal with 100,000 users daily, each visiting the site for a couple of hours?"
Yes, see above.
"Could a site like Stack Overflow run on Django?"
My gut feeling is yes but, as others answered and Mike Malone mentions in his presentation, database design is critical. Strong proof might also be found at www.cnprog.com if we can find any reliable traffic stats. Anyway, it's not just something that will happen by throwing together a bunch of Django models :)
There are, of course, many more sites and bloggers of interest, but I have got to stop somewhere!
Blog post about Using Django to build high-traffic site michaelmoore.com  described as a top 10,000 website . Quantcast stats  and compete.com stats .
Added Instagram link
Added Bitbucket & Pinterest
Added HP ePrint Center old site.
(*) The author of the edit, including such reference, used to work as outsourced developer in that project. http://www.djangoproject.com/
We're doing load testing now. We think we can support 240 concurrent requests (a sustained rate of 120 hits per second 24x7) without any significant degradation in the server performance. That would be 432,000 hits per hour. Response times aren't small (our transactions are large) but there's no degradation from our baseline performance as the load increases.
We're using Apache front-ending Django and MySQL. The OS is Red Hat Enterprise Linux (RHEL). 64-bit. We use mod_wsgi in daemon mode for Django. We've done no cache or database optimization other than to accept the defaults.
We're all in one VM on a 64-bit Dell with (I think) 32Gb RAM.
Since performance is almost the same for 20 or 200 concurrent users, we don't need to spend huge amounts of time "tweaking". Instead we simply need to keep our base performance up through ordinary SSL performance improvements, ordinary database design and implementation (indexing, etc.), ordinary firewall performance improvements, etc.
What we do measure is our load test laptops struggling under the insane workload of 15 processes running 16 concurrent threads of requests.
Not sure about the number of daily visits but here are a few examples of large Django sites:
Screencast on how to deploy django with scaling in mind http://ontwik.com/python/django-deployment-workshop-by-jacob-kaplan-moss/
Here is a link to list of high traffic Django sites on Quora . http://disqus.com/
What's the "largest" site that's built on Django today? (I measure size mostly by user traffic)
In the US, Mahalo . I'm told they handle roughly 10 million uniques a month.
Abroad, the Globo  network (a network of news, sports, and entertainment sites in Brazil); Alexa ranks them in to top 100 globally (around 80th currently).
Other notable Django users include PBS, National Geographic, Discovery, NASA (actually a number of different divisions within NASA), and the Library of Congress.
Can Django deal with 100k users daily, each visiting the site for a couple of hours?
Yes -- but only if you've written your application right, and if you've got enough hardware. Django's not a magic bullet.
Could a site like StackOverflow run on Django?
Yes (but see above).
Technology-wise, easily: see soclone  for one attempt. Traffic-wise, compete pegs StackOverflow at under 1 million uniques per month. I can name at least dozen Django sites with more traffic than SO. http://mahalo.com/
Playing devil's advocate a little bit:
You should check the DjangoCon 2008 Keynote , delivered by Cal Henderson , titled "Why I hate Django" where he pretty much goes over everything Django is missing that you might want to do in a high traffic website. At the end of the day you have to take this all with an open mind because it is perfectly possible to write Django apps that scale, but I thought it was a good presentation and relevant to your question. http://www.youtube.com/watch?v=i6Fr65PFqfk
The largest django site I know of is the Washington Post , which would certainly indicate that it can scale well.
Good design decisions probably have a bigger performance impact than anything else. Twitter is often cited as a site which embodies the performance issues with another dynamic interpreted language based web framework, Ruby on Rails - yet Twitter engineers have stated that the framework isn't as much an issue as some of the database design choices they made early on.
Django works very nicely with memcached and provides some classes for managing the cache, which is where you would resolve the majority of your performance issues. What you deliver on the wire is almost more important than your backend in reality - using a tool like yslow is critical for a high performance web application. You can always throw more hardware at your backend, but you can't change your users bandwidth. http://www.washingtonpost.com/
Scaling Web apps is not about web frameworks or languages, is about your architecture. It's about how you handle you browser cache, your database cache, how you use non-standard persistence providers (like CouchDB ), how tuned is your database and a lot of other stuff...
Don't bother... http://couchdb.apache.org/
I was at the EuroDjangoCon conference the other week, and this was the subject of a couple of talks - including from the founders of what was the largest Django-based site, Pownce (slides from one talk here ). The main message is that it's not Django you have to worry about, but things like proper caching, load balancing, database optimisation, etc.
Django actually has hooks for most of those things - caching, in particular, is made very easy. http://immike.net/files/scaling%5Fdjango.pdf
I'm sure you're looking for a more solid answer, but the most obvious objective validation I can think of is that Google pushes Django for use with its App Engine  framework. If anybody knows about and deals with scalability on a regular basis, it's Google. From what I've read, the most limiting factor seems to be the database back-end, which is why Google uses their own... http://en.wikipedia.org/wiki/Google_App_Engine
I think we might as well add Apple's App of the year for 2011, Instagram , to the list which uses django intensively. http://instagram-engineering.tumblr.com/post/13649370142/what-powers-instagram-hundreds-of-instances-dozens-of
Could a site like Stack Overflow run on Django?
Chinese version of Stack Overflow is using Django:
Yes it can. It could be Django with Python or Ruby on Rails. It will still scale.
There are few different techniques. First, caching is not scaling. You could have several application servers balanced with nginx as the front in addition to hardware balancer(s). To scale on the database side you can go pretty far with read slave in MySQL / PostgreSQL if you go the RDBMS way.
Some good examples of heavy traffic websites in Django could be:
You can feel safe. http://en.wikipedia.org/wiki/Pownce
If you haven't already, I recommend reading the section on scaling in The Django Book:
Or the newer version:
Here's a list of some relatively high-profile things built in Django:
The Guardian's " Investigate your MP's expenses " app
Politifact.com (here's a Blog post  talking about the (positive) experience. Site won a Pulitzer.
NY Times' Represent  app
Peter Harkins, one of the programmers over at WaPo, lists all the stuff they’ve built with Django  on his blog
It's a little old, but someone from the LA Times gave a basic overview  of why they went with Django.
The Onion's AV Club was recently moved from (I think Drupal) to Django.
I imagine a number of these these sites probably gets well over 100k+ hits per day. Django can certainly do 100k hits/day and more. But YMMV in getting your particular site there depending on what you're building.
There are caching options at the Django level (for example caching querysets and views in memcached  can work wonders) and beyond (upstream caches like Squid ). Database Server specifications will also be a factor (and usually the place to splurge), as is how well you've tuned it. Don't assume, for example, that Django's going set up indexes properly. Don't assume that the default PostgreSQL  or MySQL  configuration is the right one.
Furthermore, you always have the option of having multiple application servers running Django if that is the slow point, with a software or hardware load balancer in front.
Finally, are you serving static content on the same server as Django? Are you using Apache or something like nginx  or lighttpd ? Can you afford to use a CDN  for static content? These are things to think about, but it's all very speculative. 100k hits/day isn't the only variable: how much do you want to spend? How much expertise do you have managing all these components? How much time do you have to pull it all together? http://mps-expenses.guardian.co.uk/
Note that if you're expecting 100K users per day, that are active for hours at a time (meaning max of 20K+ concurrent users), you're going to need A LOT of servers. SO has ~15,000 registered users, and most of them are probably not active daily. While the bulk of traffic comes from unregistered users, I'm guessing that very few of them stay on the site more than a couple minutes (i.e. they follow google search results then leave).
For that volume, expect at least 30 servers ... which is still a rather heavy 1,000 concurrent users per server.
Another example is rasp.yandex.ru, Russian transport timetable service. Its attendance satisfies your requirements.
If you have a site with some static content, then putting a Varnish  server in front will dramatically increase your performance. Even a single box can then easily spit out 100 Mbit/s of traffic.
Note that with dynamic content, using something like Varnish becomes a lot more tricky. http://en.wikipedia.org/wiki/Varnish_%28software%29
I have been using Django for over a year now, and am very impressed with how it manages to combine modularity, scalability and speed of development. Like with any technology, it comes with a learning curve. However, this learning curve is made a lot less steep by the excellent documentation from the Django community. Django has been able to handle everything I have thrown at it really well. It looks like it will be able to scale well into the future.
BidRodeo Penny Auctions  is a moderately sized Django powered website. It is a very dynamic website and does handle a good number of page views a day. http://www.bidrodeo.com/
My experience with Django is minimal but I do remember in The Django Book they have a chapter where they interview people running some of the larger Django applications. Here is a link.  I guess it could provide some insights.
It says curse.com is one of the largest Django applications with around 60-90 million page views in a month. http://www.djangobook.com/en/1.0/appendixA/
You can definitely run a high-traffic site in Django. Check out this pre-Django 1.0 but still relevant post here: http://menendez.com/blog/launching-high-performance-django-site/
Check out this micro news aggregator called EveryBlock .
It's entirely written in Django. In fact they are the people who developed the Django framework itself. http://everyblock.com
The developer advocate for YouTube gave a talk about scaling Python at PyCon 2012 , which is also relevant to scaling Django.
YouTube has more than a billion users , and YouTube is built on Python. https://www.youtube.com/watch?v=G-lGCC4KKok
If you want to use Open source then there are many options for you. But python is best among them it have many libraries and a super awesome community. These are reasons which might change your mind:
Python is very good but it is a interpreted language which makes it slow. But many accelerator and caching services are there which partly solve this problem.
If you are thinking about rapid development then Ruby on Rails is best among all. The main motto of this(ROR) framework is to give a comfortable experience to the developers. If you compare Ruby and Python both have nearly same syntaxes.
Google App Engine is very good service but it will bind you in its scope, you don't get chance to experiment new things. Instead of it you can use Digital Ocean  cloud which will only take $5/Month charge for its simplest droplet. Heroku  is another free service where you can deploy your product.
Yes! Yes! What you heard is totally correct but here are some examples which are using other technologies
Conclusion is a framework or language won't do everything for you. A better architecture, designing and strategy will give you a scalable website. Instagram is biggest example,this small team is managing such huge data. Here is one blog  about its architecture must read it. https://digitalocean.com/
Spreading the tasks evenly, in short optimizing each and every aspect including DBs, Files, Images, CSS etc. and balancing the load with several other resources is necessary once your site/application starts growing. OR you make some more space for it to grow. Implementation of latest technologies like CDN, Cloud are must with huge sites. Just developing and tweaking an application won't give your the cent percent satisfation, other components also play an important role.
The problem is not to know if django can scale or not.
The right way is to understand and know which are the network design patterns and tools to put under your django/symfony/rails project to scale well.
Some ideas can be :
Hope it help a bit. This is my tiny rock to the mountain.
Even-though there have been a lot of great answers here, I just feel like pointing out, that nobody have put emphasis on..
It depends on the application
If you application is light on writes, as in you are reading a lot more data from the DB than you are writing. Then scaling django should be fairly trivial, heck, it comes with some fairly decent output/view caching straight out of the box. Make use of that, and say, redis as a cache provider, put a load balancer in front of it, spin up n-instances and you should be able to deal with a VERY large amount of traffic.
Now, if you have to do thousands of complex writes a second? Different story. Is Django going to be a bad choice? Well, not necessarily, depends on how you architect your solution really, and also, what your requirements are.
Just my two cents :-)
I develop high traffic sites using Django for the national broadcaster in Ireland. It works well for us. Developing a high performance site is more than about just choosing a framework. A framework will only be one part of a system that is as strong as it's weakest link. Using the latest framework 'X' won't solve your performance issues if the problem is slow database queries or a badly configured server or network.
I don't think the issue is really about Django scaling.
I really suggest you look into your architecture that's what is going to help you with you scaling needs.If you get that wrong there is not point on how well Django performs. Performance != Scale. You can have a system that has amazing performance but does not scale and vice versa.
Is your application database bound? If it is then your scale issues lie there as well. How are you planning on interacting with the database from Django? What happens when you database cannot process requests as fast as Django accepts them? What happens when your data outgrows one physical machine. You need to account for how you plan on dealing with those circumstances.
Moreover, What happens when your traffic outgrows one app server? how you handle sessions in this case can be tricky, more often than not you'd probably require a shared nothing architecture. Again that depends on your application.
In short Languages is not what determines scale, a language is responsible for performance(again depending on your applications different languages perform differently). It is your design and architecture that makes scaling a reality.
I hope it helps, would be glad to help further if you have questions.
See further details as mentioned below:
It’s not uncommon to hear people say “Django doesn’t scale”. Depending on how you look at it, the statement is either completely true or patently false. Django, on its own, doesn’t scale.
The same can be said of Ruby on Rails, Flask, PHP, or any other language used by a database-driven dynamic website.
The good news, however, is that Django interacts beautifully with a suite of caching and load balancing tools that will allow it to scale to as much traffic as you can throw at it.
Contrary to what you may have read online, it can do so without replacing core components often labeled as “too slow” such as the database ORM or the template layer.
Disqus serves over 8 billion page views per month. Those are some huge numbers.
These teams have proven Django most certainly does scale. Our experience here at Lincoln Loop backs it up.
We’ve built big Django sites capable of spending the day on the Reddit homepage without breaking a sweat.
Django’s scaling success stories are almost too numerous to list at this point.
It backs Disqus, Instagram, and Pinterest. Want some more proof? Instagram was able to sustain over 30 million users on Django with only 3 engineers (2 of which had no back-end development https://highperformancedjango.com/
Today we use many web apps and sites for our needs. Most of them are highly useful. I will show you some of them used by python or django.
Washington Post 
The Washington Post’s website is a hugely popular online news source to accompany their daily paper. Its’ huge amount of views and traffic can be easily handled by the Django web framework.
Washington Post - 52.2 million unique visitors (March, 2015)
The National Aeronautics and Space Administration’s official website is the place to find news, pictures, and videos about their ongoing space exploration. This Django website can easily handle huge amounts of views and traffic.
2 million visitors monthly
The Guardian 
The Guardian is a British news and media website owned by the Guardian Media Group. It contains nearly all of the content of the newspapers The Guardian and The Observer. This huge data is handled by Django.
The Guardian (commenting system) - 41,6 million unique visitors (October, 2014)
We all know YouTube as the place to upload cat videos and fails. As one of the most popular websites in existence, it provides us with endless hours of video entertainment. The Python programming language powers it and the features we love.
DropBox started the online document storing revolution that has become part of daily life. We now store almost everything in the cloud. Dropbox allows us to store, sync, and share almost anything using the power of Python.
Survey Monkey 
Survey Monkey is the largest online survey company. They can handle over one million responses every day on their rewritten Python website.
Quora is the number one place online to ask a question and receive answers from a community of individuals. On their Python website relevant results are answered, edited, and organized by these community members.
A majority of the code for Bitly URL shortening services and analytics are all built with Python. Their service can handle hundreds of millions of events per day.
Reddit is known as the front page of the internet. It is the place online to find information or entertainment based on thousands of different categories. Posts and links are user generated and are promoted to the top through votes. Many of Reddit’s capabilities rely on Python for their functionality.
Hipmunk is an online consumer travel site that compares the top travel sites to find you the best deals. This Python website’s tools allow you to find the cheapest hotels and flights for your destination.
More here https://www.shuup.com/en/blog/25-of-the-most-popular-python-and-django-websites/
Can Django deal with 100,000 users daily, each visiting the site for a couple of hours?
Yes, but use proper architecture, database design, use of cache, use load balances and multiple servers/ nods
Could a site like Stack Overflow run on Django?
Yes, just need to follow answer mentioned in the 2nd question