Earlier quoted context omitted.
Switching social networks is not the same as switching email services. You don't just up and move. Millions switched from MySpace to Facebook but what they really did was join Facebook while still using MySpace. And then using MySpace less. And then not at all. So, in theory, people could "join" Google+ because they see it in the toolbar while they're searching, and if there are a few killer features that bring them…
MySpace wasn't prepared for Facebook, and ignored it until about 2009 when it was too late. People saw their friends on Facebook, so they stopped using MySpace (hence the network effect I mentioned earlier). Facebook /will/ fight for its social dominance, but (generally speaking) there is a site out there that they don't know about and won't be prepared for (aka "the next Facebook")
Care to illucidate us?