Facebook is a walled garden w/ no anonymity. Privacy (specifically, most posts are only seen by friends of poster) can be good but it can also stifle discussion. Google Plus might have been good (and got some notable adoption among academics initially) but could never accumulate enough of a network for a variety of reasons. Reddit actually has a lot going for it for these purposes, but doesn't work well for "personal brand" building, so doesn't get much uptake among experts.

Other social networks I can think of are mostly focused on sharing pictures and experiences among close friends.