It's a dream job for developers, in some ways a lot more so than the big boys like Google and Facebook. They have a hiring pipeline any tech company would kill for. They probably don't have the deep security talent that say Google or Microsoft have, but they should have enough.
Really? I'm not so sure, AFAICT Github doesn't have any new or interesting problems to deal with. It's just a Rails app that's constantly developed on. You can do that, well, anywhere.
Having met several of them and spent a day in their offices (they gave Kiva engineering a tour day a couple years back), I can say they have an awesome company culture and space and great leadership and great brand recognition. Plus, people generally like them. I'd put it near the top of my list if I had one.