By their very nature, git repos are one of the easiest things to migrate. Simply point at a new remote and push, and that's really it. It means that, unlike many other services, I could see GitHub being completely abandoned almost over night. If something better came along.
True, but thanks to the API and their relatively simple structure it's reasonably easy to at least copy their contents as well. Linking them correctly to user accounts on a new platform is probably the biggest issue.
