There a massive concern for security when dealing with social networking sites such as facebook. Since Websense aquired Defensio, they have been working hard on v2 of the product. Free to users, Defensio 2.0 will monitor your facebook site for malicious posts, spam and viruses. The application cross-references content posted to users pages against their correlated threat databases. If a threat is detected it logs it and advises the end user who then makes the decision to block or not. I would imagine this would then feed into a Bayesian database for a smarter for automatic protection mechanism.

You can sign up for the service and the Defensio website.