Maybe it's that once a platform achieves a certain size then the potential of making real money comes into view, and that starts to be the objective to optimize for. That means employing typical short-sighted stock boosting strategies such as explicitly focusing on revenue (Digg), or targeting mass appeal. Only the masses do not produce very good content, and they overwhelm the initial users in numbers by an order of magnitude. So the quality of the platform goes to shit.
