G+ was good (a lot of academics were using it heavily for a while and I think John Baez, for example, still is), but it never accumulated much of a userbase (the reason I don't spend much time on it and probably the same reason you don't).

I don't think the character limit is a boon. Sina Weibo removed it, and that seemed to go well.

edit:

One thing I forgot to mention: I use Twitter lists a lot and have various ones assigned to different fields (econ, genetics, politics/current events, microfiction, etc.). Twitter search can filter by list, so, eg, I can see any idle comment any of ~70 economists has ever made about: real business cycles, endogenous vs. exogenous effects, moral hazard, regression discontinuities, etc., at least on Twitter. I can't really do that on Facebook (no real search, right?) or G+ (might have something similar but not enough people using it).