It's a bit more nuanced than that. One of the social sites I briefly used counted clicks more then upvotes. Predictably, being a 30ysh year old male, I usualy clicked on links promising pictures of (more or less necked) women. In less then two weeks most of my personalized content was either porn or lolcats.

The moral of the story: while I do enjoy the ocasional porn and lolcat, it's definitely not what I want from the net (also why I didn't use the site for more then 2 weeks). It's just that it was much easier for their algorithm to categorize those things then the much more abstract "interesting", or even the somewhat less abstract "programming".

edit: It's not that social sites are doomed. It's just they're damn harder to get right then just digg, and they're still in infancy.

Yeah. I have high hopes for social sites. But it's why I prefer sites trying new models (like what Facebook was like before it went overboard) rather than sites that try to be "like Digg but for X."