I personally find it hard to fathom that when my generation's kids grow up they will want to virtually 'hang out' in the same place their parents do or did.
True, I have some aunties and uncles who have Facebook accounts, but they are in no way as heavily invested as my peers. They really only log on once in a while.
Kids want to go somewhere where to be young and do stuff with their friends away from the gaze of their parents. And that's going to be difficult to do on Facebook, when the Facebook generation are their parents.
Also the idea of your kids being able to see every picture and everything you wrote when you were young seems a bit weird. Some people will say: 'yeh but my kids are like my friends, we tell each other everything.' But I think they'll be in the minority.
I think when my kids grow they be spending their time on something I've never heard of. If that happens Facebook's probably going to be in the way out.
That's my reason, I guess