I have spent the last two hours trawling information, legal web sites, court reports, hearsay comments on numerous forums, Forum Administration Management government advice information and advice articles from Copyright Lawyers in Australia, USA and UK and can find not a single case of a public forum, administrator or poster being sued for copyright infringement.
In the USA by regulation you can not be sued if you provide a contact means on your site for the copyright owner to complain and remove such material as soon as requested. This does not protect those stealing stuff for commercial purposes.
I can find several cases of commercial sites and individual business sites posting, claiming ownership or otherwise profiting from misuse of copyright material. All were for commercial purposes to mislead or create an income from someone else's work.
The only chance of trouble for an Administrator/Owner/Poster on an open non-commercial forum like this one is for libel or slander (naming-and-shaming, false reputation claims or other personal attack, misleading information designed to part people from their money or damaging advice such as pseudo medical diagnosis). None of this applies to our well controlled situation. There have been some very high profile cases where well known people have both won and lost big money payouts for personal attacks.
The recent changes to the rules making sites and managers liable for posts is aimed at curbing uncontrolled libel, slander and personal attack or a reputational or political campaign and the copyright question is only collateral damage and not the intention of the law.
This just reinforces my opinion that regarding copyright infringements there is about zero chance of legal action (lawyers nasty letters) let alone any form of payout judgement. It would be interesting to know how many copyright complaints have arisen in the tens of thousands of posts that have been through this forum or if anyone has documented evidence of a copyright complaint resulting in actual court action for the hundreds of millions of posts on other similar community forums.
I won't make any more comment.