Just an addendum to the Github numbers, I hadn't realized Adrian Holovaty had renamed the previous Github repository to django-old, replacing it with the brand new repository (with the SVN history, etc).
So if you look at django-old's numbers: https://github.com/django/django-old
Watchers: 4,030
Forks: 753
compared to Bitbucket's:
Followers: 266
Forks: 39
So the raw numbers are dramatically in Github's favour.
Beyond the "key user" metric, the raw numbers also seem to lean in favour Github: there are 665 watchers on Github, 266 followers on Bitbucket; 56 forks on Github, 39 on Bitbucket.