Github isn't a social network in the way that traditional online social networks have been viewed – it isn't just about "connecting" and communicating the way that friendster, facebook and twitter all are.
Instead, it's an online code repository that has social features – in other words, it's the next step in making our online selves a more effective extension of our off-line selves, doing work, building things, but doing it in the context of a social group – just like we do in the off-line world, more often than not.
This is an example of a niche concept becoming widespread enough that it almost becomes table stakes rather than a notable feature.
That's still pretty cool – but again, I think it means that the term "social network" is losing its distinctiveness.