That's mostly irrelevant. A lot of people are not using Github just because it supports git - the network effect is one of the key drivers in choosing Github.
Certainly but that wasn't on OPs list of reasons to use it :)
Looks like it was:
> Why Git/GitHub, as opposed to Mercurial/Bitbucket or some other system? Because it's very well-made, and it's where the people are.
Where the people are would imply because of the social aspects.