An interesting article (in French) by Laurent Jacobs indicates that a recent study from Rubicon Consulting explains how 80% of user generated content comes from on average 10% of the online population.
There is also an interesting piece about moderation and how this needs to be quickly managed or be managed after content is posted in order to prevent turning away the key contributors. Sorry for the non French speaking people I haven't got time to translate this one...
Note: check out this article, posted after I talked about the above article