Ok, I'm starting to become one of those people who thinks Google+ might displace Facebook. It sounds insane, especially to me. Facebook is entrenched, and holding the definer's perch. They are the dominant player, and have really set the stage in this arena. Others react to them. Until now.
Google is much more intuitive. Much slicker interface. Not so cluttered. These may or may not unseat Facebook, but they're innovating and Facebook is in the position of reacting.
Now, I don't expect a sudden exodus. First, many hardcore Facebook types spend hours with their Farmville, gang war, etc games. Until the the equivalents arise on Google+, all bets are off. But, even now, Google has established a place. Perhaps not as great as it could be. But still with a large affect. And who knows? They may just birth a whole new genre. Who knows...
The social media game is shifting under our feet. That's what it does. Might even say that's what it IS. It will be fun seeing how this flows from here.