Another perspective on the “Google+ is to Facebook” analogy; also check the Slate “Google+ Is Dead” piece
As Bing is to Google, so Google+ is to Facebook. Google+ is Facebook's Bing. It might not ever be as big, nor even rival the numbers, but being second place in a field of two can be nice.
It can especially be nice when unlike Bing, Google really doesn't have to toss away tons of money to build marketshare. No doubt, Google+ on its own would be a loss-making exercise right now. But as it helps Google shore up its weak social graph defenses, it'll probably prove well worth whatever's being spent -- even if it stays in second place.