The biggest feature that Github provides is its popularity as a central meeting ground for project discovery and collaboration. Some people are only looking for git hosting and don't care about the community aspect, so they could jump ship to Bitbucket or Gitlab.

In that respect, if you're looking for a social network to keep in contact with a general group of people, Facebook is the way to go, and not the myriad of smaller sized social networks. If you're only looking for communication with a small group of people who haven't committed to anything, then you could jump ship to Google+ or whatever. Size of community is the salient feature, not the app.