Personally, I think the reason for Google+ 's inability to gain full momentum is thats libraries already have facebook and twitter. Most of them took way to long to move to those social networks, googl+ is the same. I think in time they may transfer over (at least the new age, innovative ones). Similarly, when twitter and facebook bloomed most of the librarians in that period of time didn't want to move to those social networks at all, and only the new age innovative ones hopped in.
I do most of my discussions through Twitter -> which then post to facebook, linkedin, and Google+ . If I wanted something bigger as a discussion, I use my blog which posts to twitter which repeats the above. Idk if thats what you were asking or looking for