I think social networks are cyclic because they have a 'fashion' element to them. Whereever the 'hip' people are that's where the rest will follow. So once the 'hip' people (some would say the trend-setters) move out their groupies will follow.

Of course after that it's a long way down that hill that you can literally shoot up on but I don't think there ever will be a permanent social network. It's more like a locust thing. And hip people will always be on the move, if only because they really can't be seen eating in the same restaurant that every other person eats at, so they'll go and discover a new joint that nobody else has heard of or a new style of wearing their hair, their jeans or their baseball caps. It's a fashion thing.