Google Plus is projected to hit 400 million users by the end of 2012. Some are even suggesting it may overtake Facebook.
Sign Ups Don't Equal Engagement
I have a theory... everyone is signing up to Google Plus because, let's face it, everyone is on Google in some capacity, probably most heavily Google Talk and Gmail. But, when push comes to shove, they are going back to the tried and trusted Facebook to keep up with their social connections.
The Bigger Picture
It has been a thought for a while that Google is less concerned about the day to day posting as a network, but more about the social element that having a platform like Google Plus affords. Want to use Hangouts? You need a profile. Want to recommend content so that it features higher in Googles new "Social Search?" You need a profile. Google understands that a social element is essential to keep their product set relevant in this socially connected world, but not sure the play has ever been to simply "rival Facebook."
Opening the API
One thing the folks at Google haven't done yet? They haven't opened the API for developers to integrate (except a few chosen companies in a very beta way). Once apps can integrate with it fully (not just post to it like plugins allow) we may see a major spike in usage.