Facebook itself was third to market in social networking, after Friendster and MySpace. The latter was heavily entrenched and very successful when Facebook started unseating it.

G+ definitely suffered by coming to market after FB and Twitter, but that's not the whole of the story. More specifically, it came to market after Facebook and Twitter, without doing anything fundamentally different, better, or more useful than Facebook or Twitter. (Circles, the putative differentiating factor, added more of a burden than a solution to the standard social networking UX.)

Late entrants can sometimes win; Google and Facebook were pretty far from the first movers in their respective markets. The key is that late entrants need to be significantly better than anyone who's come before them. G+ was not.