And what I've noticed is that all successful social network sites are structured to provide an attractive nuisance.
This isn't to say that they aren't sometimes useful, but in order to attract users, a social networking side like Facebook or LinkedIn has to keep folks coming back. It's not enough to get them to create a user ID in the first place; I've seen some estimates that around 90% of legitimate, human-derived accounts on social networking sites are inactive. (I qualify this as human-derived because a whole lot of them are bot-generated accounts used by spammers. I'm talking about the ones with a human brain behind the name.) So the successful sites need to get real humans to keep coming back — especially if they're going to raise the advertising revenue from click-throughs to pay their bandwidth bills — and the developers are therefore subjected to a ruthless Darwinian selection pressure: add attractive nuisances, or die.
We can see this on FaceBook with its endless games. (I sometimes wonder if I'm a Facebook widower.) We can see this on LJ with its endless rounds of emotional affirmation in comment threads. We used to see it on USENET back in the eighties and nineties, with the flamewar season. Social networks don't grow because they provide utility to their users: they grow because they keep pushing the social stimulus button.
I do get something out of Facebook (by disabling all games and other intrusive apps), and a little bit out of Twitter. But in general, I think Charlie's spot on.