I think it's too early to tell. They're really shoving it down our throats with deep integration and social search returns. They're trying so hard to brute force it, and I'm not sure that they won't eventually succeed. Chances are they won't, but I'm just not convinced that we can call it over yet. I think this is going to play out over at least two more years before we'll know if it's gonna stick around in any sort of meaningful way.
The problem for google is that people's parents and grandparents are on Facebook. I mean, when they've penetrated that deep demographically, how are you going to yank them out into a new platform? Early adopters can only do so much. I think the most likely scenario for G+ (if it survives) is that it becomes a different social network alongside Facebook, LinkedIn, etc that serves a niche community.