The only thing I use Google+ for is bookmarking. It's my bitly. I have g+1 button plug-in installed on my browser, so whenever I see something interesting I bookmark it using it. Would this count as active usage? Probably. But am I actually an active user? No. I don't go to their front page everyday, check the news feed, e.t.c. So the 100 million is probably an over-estimate.
