I doubt Git would be popular without GitHub. Had the GH founders instead been more familiar with Mercurial, we would all be using the latter. Having used both extensively, I think we would all be better off in that alternate history, but such is life.
As far as I remember, Git was already going strong at the time GitHub appeared. I was also a supporter of Mercurial at first, but I think Git won because:
* it always was fast (written in C vs Python for Mercurial);
