That's not how it works, having copyrighted content isn't proof. You have to 'knowingly infringe'. As long as FJ took down content when someone said 'hey this is copyrighted' then he hasn't done anything wrong in the eyes of the law.
Good news. I don't really know what this IMVU stuff is all about, but it's terrible that the people in charge aren't even trying to regulate or protect content, especially if user generated content is supposed to be a big part of their business.