While amusing... the value of most of those sites is not the site themselves. But the people network that is there. Why is everyone using it? Because everyone is using it. Then suddenly the 'crowd' will move to the next one. There will be a few holdouts that stick but it will be a zombie site that lumbers on. For example myspace, digg, and fark are still around.
I think you will always find a core that stays and a core that leaves.. the reason is that people don't feel heard so they look for other places to voice themselves. I went from slashdot to reddit to HN. Even now HN doesn't feel satisfactory.