Just a datapoint, but out of all the invitations I sent out when G+ first launched (100-150, I don't recall), 40 or 50 friends signed up. ~3 of them still use it at least once a week.

From my observations, G+ still seems to consist primarily of three major user groups:

  1. Tech celebrities
  2. Technologists in general
  3. Non-US users

Facebook's feature set is killer, but at some point, I really would like to use some social software that doesn't silently launch a blinking GPS signal when I'm reading my news feed (I know, I can disable it, but still...).