Despite a strong initial start, in the universe of social media it's widely believed that Google+ will never replace Facebook.
That said, I'd like to get all Thomas Friedman-y and speculate wildly from a very limited personal anecdote. Namely, my 6th grader just begged to be allowed to join Google+. Begged! Stomping and huffing was involved when the request was denied. And why? "All my friends are on Google+!!!!"
I know for a fact that my kid's friends have parents who would certainly refuse to let them open an account on Facebook. My theory is that Google+ is such a "failure" as a social media hub that it's possible for parents to not even know that it's a social media hub. Or, if they do, the existence of "circles" as a way of limiting exposure gives parents a better sense of control, making Google+ seem like a more appropriate first step. And from the kids' point of view, Facebook is for grown-ups. Would you want to be on the same social media channel as your mom??? Ew. Plus, because it's unpopular with, you know, old people, you don't have to worry about parents checking your status ten times a day.
In short, I'm starting to wonder.
Edit: And, look what just showed up in the news: "Google+ outranks Twitter as Number 2 Social Network after Facebook"