There's definitely an argument that GitHub is one of the primary reasons that Git beat Mercurial.

Anecdotally, I started using git because of projects on Github I wanted to contribute to. A number of others I know where in a similar boat. Before that, we used subversion, bazaar or mercurial. I personally am happy with having been pushed to using git and if it was winning anyway (not clear) I'm sure I would have eventually ended there anyway, but GitHub is the reason I started using it when I did.

Yep!

"One click" fork + "one click" pull request are its killer features.

I've used Mercurial. It sucks compared to Git. And not because I can't use it on GitHub.

It's tricky to compare them now, because Git won, it's got a lot more investment. Ideally you'd need to care them just before Git got the upper hand - but that's hard to pinpoint.