I know that many librarians are concerned about having social media sites (such as blogs, Facebook, Twitter, etc.) because people can leave comments that they may not be able to moderate before they're posted. This article from CNN, called "News sites reining in nasty user comments," discusses what various news agencies have been trying.
There are no magic bullets yet, but the article gives you a taste of various attempts at solutions. Keep in mind that it's about news sites, not library or nonprofit sites. The news definitely attracts different sorts of readers and commentary. As proof, check the many comments at the end of this article, both for and against allowing people to comment anonymously.