This effect seems to happen commonly among news aggregating site communities.

Digg used to be pretty cool when it was used largely by people excited about the start-up/tech enthusiasts. As it became more popular, the user-base seemed to devolve as the content started to cater to the LCD.

Reddit has done a good job combating this, with their implementation of subreddits that focus on specific core user demographics. But most of their largest subreddits have a dubious quality of users.

Hacker News seems to have the best quality of discourse/users at the moment, but I attribute this to its relatively small user-base.