Never heard of it, but if you really have witnessed anyone confess to a rape or state plans to rape I hope you passed the info onto the police?
While these groups should be monitored by the police you would think, a confession on a fantasy site means nothing as evidence, unless you have their location, and the person in question is already a suspect, or they give enough detail to connect themselves to a rape under investigation.
Police can track people down, but they probably aren't going to use resources to do that, for someone that is likely in a different country or where they can simply say that their confession was merely a fantasy.