Sure, I can understand that. But that is based on the notion that Github will stay the way it is, simply a place for collaboration. However, it can amount to so much more, it can spin out its own careers site, like stackoverflow did. The point I'm trying to make is that Github has ludicrous potential, and if you value it based on that 2bn seems rather low.
I don't think Github can get away with job advertising like StackOverflow. I pay Github money; if they start advertising to me I'm leaving. Free sites can advertise at me all they'd like.