Broadly speaking, if the statement can be substantiated as true,...

Broadly speaking, if the statement can be substantiated as true, and was made in the public interest it's a defence to the claim of defamation.

Plus damage must be proven which means that the statement must have affected a significant number of the target audience causing a loss of reputation.

So if you have statements which are true, and made in the public interest, it's a defence, even if damage has occurred.

But if you haven't even got damage, because so many aren't even listening...weeell.
Not so much of a case is it?

(BTW anonymous posters can potentially defame a known public figure, and they can defame eachother if they are known to eachother. This is to be distinguished from situations where everyone is anonymous. Worth noting surely?).