There was the "(name of kid I dont remember) is a h*mo and should die" group, the "shut up dumb b*tches" group and the "(name) is a dirty wh*re" group.
I don't say this very often, but this is very worrying to me. This is IMO is not typical bullying behavior. I had a really rough time with being bullied in school and I never experienced anything like this. If it was something more harmless like cursing on his page, or mildly insulting people, I'd probably just tell you to keep an eye on it. As it is, I think you need to speak to his parents, and his principal (presuming these groups are directed at schoolmates). If it was just your cousin, I wouldn't necessarily suggest going to the principal so quickly, but presumably there are other classmates joining/creating these groups and that increases the scope of the problem.
Please understand that I am not saying your cousin is some terrible person in general, or that he won't grow out of this, but this seems to be outside the norm for teenage behavior. Its bordering on the potential
for violence (not just your nephew - his friends who participate in these groups as well). Additionally, thank you for reporting these groups to facebook.
Edited because its "cousin" not "nephew"