As I mentioned earlier, building a website based on user generated content is somewhat like a potter making an earthen pot. Since there are so many websites coming up that rely on the user generated content, it is imperative to understand the process that can make these websites successful or an utter failure.
The pot is made by the momentum of the wheel. And what does the Potter do? Gives shape to the clay of lump so that an earthen pot comes out of it.
First he applies hard force so that something starts rising from the clay lump. And then slowly and gently, control the upward and sideways movement of the clay as it rises further and further up.
Similarly, for the website, first ensure that the wheel is spinning i.e. there is willingness in people to contribute to the cause you are standing up for. Put in some hard work (may be hire some people) to create a critical mass of information. This will set the momentum in the right direction. And then slowly and gently, keep things in order while people dump more and more info to your site.
Of course, things are not so simplistic but there is certainly a pattern.