I've always thought that social networks like Friendster, MySpace and Facebook follow a clear and old path laid out by the fashion industry.
Fashionable things are almost by definition meant not to last.
The early adopters will often take up something, be it Facebook, or new clothes, or a new band, largely because it is not (yet) popular. Or because it is exclusive.
Remember when Facebook was for just for ivy leaguers?
I remember when years ago Joel (of Joel on software) wanted a Facebook that really is strictly restricted to college students. Remember that?
The late adopters will often follow the early adopters for no other reason then because the new thing is cool.
And this pattern guarantees there's always something new and nothing is cool for too long.
Now there's been a lot of talk about lock-in, social graphs, walled gardens, etc.. all reason why no one can do to Facebook what Facebook did to MySpace.
But.... I have my doubts. Google+ vs Facebook, I'm going to get some popcorn and enjoy watching this.