What Facebook has over all of the other players is the network effect. Metcalf's Law tells us that everyone is on Facebook because... well, everyone is on Facebook. Yes, when Facebook upset its user base back in September, lots of people created Google+ accounts which are now dormant. BUT... those people now have Google+ accounts. They're not going away. Every time Facebook does something to irritate its user base, more people will create Google+ accounts out of frustration and a desire to "stick it" to Facebook. This has the potential (but is in in no way guaranteed) to iterate enough times that it could reach a tipping point. I don't think it would tip without two things happening: 1.) Google+ changing its experience to make Facebookers feel more at home, and 2.) Facebook doing something boneheaded that infuriates lots of people to vocally switch (after the critical mass has been assembled on Google+).
I think it's very possible, albeit not terribly likely, for us a few years down the road to say: "Facebook who?" Who would have thought that Myspace would have been unseated?