Critical mass is hugely important, but it can also be surprisingly fickle. I remember when MySpace hit the tipping point and it seemed like Friendster became a ghost town overnight. A few years later when Facebook opened up outside of colleges, MySpace turned into a dead zone.
It's possible for the same thing to happen with GitHub, though Facebook's continued entrenchment is also a signal that it may not.
Facebook is entrenched still, because they deliberately tried to avoid the mistakes that MySpace and Friendster (et al) made. They grew and expanded, changed their feature set to ensure that Facebook had everything your average person needed. GitHub haven't been doing that so much, but hopefully this response is the beginning of a change in that direction
It's possible for the same thing to happen with GitHub, though Facebook's continued entrenchment is also a signal that it may not.