If it helps, I'm a data point who's seen lots of Google+ activity among close and remote friends, embraced by many who were avid Facebook users (I wasn't). I can't compare it with others from my point of view since I joined other social networks late, but it seems to be doing quite well with people I know. The much feared group inertia was given a quick and solid kick by Google+ in my case.

Just thought I'd balance the comments about it being empty and inactive.