I'll confess that I feel that github will eventually wane. But, I can't say why I think that right off. When they run out of money? Start failing at the hosting portion of what they do in favor of other things?
Not just ads. Bundling malware into installers of popular projects.
Book: Innovators Dilemma
I'm not saying I'd bet money on Github being a front runner in say 5 years, but it isn't that unlikely for them to still be the site in 5 or 10 years.
GitHub's major selling point is mindshare/popularity, and git by default makes moving to another git hosting service relatively trivial. Yes, there are issues to transfer, but it's nowhere near as difficult as in the days of SVN, where you'd need to have shell access to dump the repo, or hope it was a modern enough svn version to support remote dumps (and that they didn't time out).