Being as I am not a member of FaceBook ummmm I cannot help apart from suggesting you contact FaceBook Administrators and discuss problem with them.
Is hacking fairly common on there ?
I think that's what you do but I'm not 100%... I'm sure that's what a friend of a friend (one of those tenous links) did. I found the above though when I googled. I guess it depends as well if a crime is being committed on FB, cyber bullying or something... I should never have strayed in to technology!
I would just phone my local station for advice then as well. They should be able to point you in the right direction at least and then maybe could tell you what sort of evidence you would need.