So we know Google has penalized sites that accept user generared content, such as Mozilla, BBC and Sprint. They even have a help document on the topic.

One Google Webmaster Help thread has a forum owner asking why his forum received a pure spam notification over a user-generated content spam notification.

This was his question:

I find it funny, somewhat, that he is complaining he got one penalty over another. Clearly, Google thinks the site is pure spam, not user generated content spam. I guess one is a lesser evil.

Forum discussion at Google Webmaster Help.