Actually it wasn’t; by claiming to be this other girl they were committing fraud (check out the facebook terms of use).
By posting the material they did they were slandering/libeling her.
I don’t think it should be viewed as “First Amendment Rights.”
First, I don't believe it is fraud. But, argumentum, let's say you're right. Then, charge them with fraud, not "cyberbullying" whatever the hell that is.
"By posting the material they did they were slandering/libeling her."
Terrific. It sounds like a great civil case to be brought. But, in this country, we don't litigate slander/libel suits in criminal court, do we? The Founders had a reason for that. Under King George, slander/libel cases WERE litigated criminally, at least those against the crown.