Google have been tremendously coy about what their success metrics for G+ are, though they've played highly disengenuous all-but-utterly-fake numbers games in playing up "engagement" since the very beginning.
I'd argue that the issue isn't numbers, but relevance. G+ is lousy in many ways but has a few small areas of success, notably its Notifications mechanic, a community which, for me, works fairly well, and a search which while pathetically under-featured is comprehensive and fast.
Winning the numbers game for social vs. Facebook in its current incarnation is a fool's errand. Numerous people have pointed this out, including ex-Googlers pointing at the "Interest Graph" (though suggestions for following / pursuing this date to the first few months of G+).
If Google does grab the Cosmo crowd, that's fine, so long as it doesn't also chase off the Nature/PLOS crowd in the process. Unfortunately, Google's proven more than happy to sling absolute snot (as in the G+ "What Snot" feature ... oh, no, that's "What's Hot").
Power users learn how to disable that instantly.