The 1-9-90 rule applies. 1% create content. 9% actively interact with it in various ways. 90% lurk. You get 25% of the 1% to leave/quit, the 9% have less to interact with and the 90% start to lose interest. It starts a downward spiral with the 1% and the site begins to crash.
Twitter is starting to see that happen with increasing losses of users.