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.
problem 2: subscription sites don't work, because they attract too few users
startup idea: a subscription voting site, with smallest possible monthly fee ($1 pcm?) - the purpose isn't to make money, but to limit users. Would probably work best for a vertical market or special interest group.
They give other reasons, but I wonder if it is also retaining the quality of the community. Also, they seem to use public bookmarkin ("favourites"), and not voting as such. Maybe that makes a difference.