That's not the only reason people use Github. It's the Facebook of programmers in some sense and it's got a critical mass that switching to something else is not practicable (not without some serious momentum).
Business Insider agrees.[1] Github, Inc. is now valued at $2 billion.
Right. Which means we have to worry about Github's management starting to think like Zuckerberg: "All your life is belong to us."
[1] http://www.businessinsider.com/github-raises-200-million-wor...
There are other Git services. There are other source control mechanisms. There are other ways to do things. Just like you can actually leave Facebook, feel free to leave Github. Or stay. But make a conscious choice.