Dare Obasanjo has done a great summary of the issues with "social media" sites:

Social news sites like Reddit & Digg also have to contend with the fact that the broader their audience gets the less controversial and original their content will be since the goal of such sites is to publish the most broadly popular content on the front page. Additionally, ideas that foster group think will gain in popularity as the culture and audience of the site congeals. Once that occurs, two things will often happen to the site (i) growth will flatten out since there is now a set audience and culture for the site and (ii) the original crop of active users will long for the old days and gripe a lot about how things have changed. This has happened to Slashdot, Kuro5hin, Reddit and every other online community I've watched over time.

It's not a new problem, either. Back when we all followed usenet news groups we saw the same progression over time.

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