I tend to think about it like the Cambriam Explosion: it seems to me that at times a new technology comes along that is so much better than anything before it that for a while it's adaptors have a kind of free ride, untill a winner evolves. My personal favourite example are computer games: it seems most kinds of games were already being written as soon as home computers were available. Eventually a few genres emerged that crushed all the other ones. "Doom" was another such leap that triggered an armada of First-Person-Shooters.

So now for a while everything "social" will survive for a while, untill the big players have manifested themselves.

Anyway, I didn't want to make a point for iTunes, just "prove" that social networks are not the only kind of applications successful today.