UK libel law
Many years back I ran a website that libelled a very large number of organisations (a recruitment consultant review site). Fun as it was, it got a bit hot and I let it lapse, but the legal advice I got at the time was that in order for it to be libel it has to be
2. published and communicated to a third party (i.e. someone other than you)
3. Must be defamatory - that is they must refer to them directly, and cause damage to their reputation amongst their peers.
In your defence you can argue one of those points, or you can argue it was "fair comment" (as in it''s an oipinion based on true facts made in good faith and without malice, and is in the public interest).
In the UK at least, the law that covers this is the Defamation Act 1996, and a notable judgement was Godfrey v Demon, which found Demon liable for a libellous comment from a third party. The key points were a) if notified of a libellous comment you must remove it, and, as I recall, b) if repeated libel is occurring and you are notified about it, you should take some steps to prevent this happening.
I'm not a lawyer but that was the advice I got from someone who was. I would expect Peeple (missing an "i" there) to be on the sharp end of most of these points fairly shortly.
Given the three pillars of the internet are spam, pornography and abuse, it was only a matter of time before someone tried to commoditise the last one.