A blogger recently failed at the High Court in his attempt to sue a commentator on his blog for defamation. The nub of the judge's ruling was that the fact that the blogger left the 'offending' comment on his site for a considerable time, mitigated against the claim that the comments caused 'substantial upset and distress'.
The implication here is clear. If you feel that comments on your website are defamatory, you should remove them asap (common sense really) - this is particularly important if someone else complains that comments on your website are defamatory to themselves.
So while social networking type sites, which allow users to add comments and their own content, are vulnerable to potentially libellous or defamatory material, the risk can be mitigated by careful policing of content and prompt action.