1. "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:
  2. Instagram : What Powers Instagram: Hundreds of Instances, Dozens of Technologies.
  3. Pinterest : Alexa rank 37 (21.4.2015) and 70 Million users in 2013
  4. Bitbucket : 200TB of Code and 2.500.000 Users
  5. Disqus : Serving 400 million people with Python.
  6. curse.com : 600k daily visits.
  7. tabblo.com : 44k daily visits, see Ned Batchelder's posts Infrastructure for modern web sites.
  8. chesspark.com : Alexa rank about 179k.
  9. pownce.com (no longer active): alexa rank about 65k.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.
  10. 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 (*).
  11. "Can Django deal with 100,000 users daily, each visiting the site for a couple of hours?"
    Yes, see above.
  12. "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.

(*) The author of the edit, including such reference, used to work as outsourced developer in that project.