FB is an example of a system where all content is there due to users, but it's profitable. On the other hand, at least from what I could see, FB never really promoted any kind of special culture or community ethos.

based on that, at least two good ways to make a community-based site profitable would be to use it as a platform for this party profit - allow others to embed profitable apps/games/... and take a percentage of their income, and sell ads based on people's interests.