No numbers to back this up but Facebook has a lot of that 40-60 year old age group locked up and they won't be the type that jumps from one social network to another. And people like my brother who finally got onto Facebook in the past year because he figured it might have some staying power. They aren't going to be jumping around to have the next and newest bells and whistles when it comes to social networking, they are just looking for the basics that Facebook already provides.
That said, Google+ is opening invites back up. Does anybody out there care to invite me? email is
[email protected]
I did read somewhere that Google is going to be a bitch and take inviting privileges away from those that invite too many but I'm not sure what that means.