Federal law protects online publication of defamatory posts made by others
A
recent ruling
by the California Supreme Court intepreting Section 230 of the
Communications Decency Act highlights the differences between online
publishing and traditional print publishing.
Web sites with reader comment, forums and story chat features are
protected by the law that essentially says the poster, not the provider
of the service, is liable for what is written.
It's a good idea to point that out in your terms of service when inviting readers to participate in an unmoderated forum.