As I understand it – typically you cannot be sued for items posted on your forums by OTHER people as long as you did not actively moderate the thread, post in question.
(Basically – you didn’t know about it)
BUT you do have a responsibility for moderation – and if you are informed of defamatory information on your website (including your forum) then you do have a legal requirement to act. Ignoring it – could then open you up to action.
This is the defence that websites like YouTube.com and Blogger.com and the like have regarding copyrighted content appearing. They are conduits – and not immediately responsible – but they have to act when breaches are pointed out.
I got this understanding from a couple of techy podcasts I listened to within the last six months where they were talking with journolists, bloggers and lawyers and they made a specific distinction between content on your site / and user content on your forums etc.
Hope it helps.
:hattip: