This situation is reminiscent of websites hosting copyrighted material posted by anonymous users. Does the relevant law (DMCA, I think) say anything about defamation? A reasonable compromise might be that websites are held harmless so long as they respond to official defamation complaints in some prescribed manner. (Of course, this might not work as well because defamation is significantly less clear-cut than copyright violations, which already has plenty of cases of over-reach due to disputes over fair use, etc.)
DMCA is for copyright infringement, not defamation. That's not to say it doesn't get abused in all sorts of "we want you to take X down" scenarios, however.
For safe harbor here, you'd be looking to the CDA, I believe. In that case, what matters is the level of editorial control you exercise over the site. The complaint alleges that they review every comment before posting, which if true, would seem to be a problem for them. My understanding of the CDA is that, somewhat perversely, you're better off having low standards than high ones with respect to user-submitted content.
That aside, the above is a layman's understanding of the relevant law, not legal advice. If you're ever in a situation where this information is important, please discuss it with a lawyer.
