The daughter of friends of mine received a stream of obscene and insulting messages via Facebook. Her dad went to the police and was told they could do nothing about it unless they know the source. The particular policeman happened to be a bit of an IT specialist and very helpful, said, as above, that the user details would only be available to the site administrators and that to get the necessary international action to divulge would be lengthy and time consuming.
His final piece of advice was 'get her off bloody Facebook and block access to it through your router's parental controls.'
Exactly!