It sounds as though you have a number of claims you could bring against your ex-fiancee. One potential problem is whether or not you can prove that the person who has logged into your Facebook account was actually your ex.
The type of behavior you describe is worthy of discussing with an attorney ASAP in order to avoid any further damage to your reputation. At the very least, you may be able to go to court to obtain a restraining order.
As I said, you really should discuss your situation with an attorney.
A crime to break into a computer (yours, or at the hosting company via false log-in), so that can be reported to the police. Then, there are likely torts (personal wrongs) here which could support a lawsuit. I agree with my colleagues here that the first step if to consult an attorney in your locale. Your attorny can advise you both as to making a police report and/or filing a lawsuit.
We do not have a client/attorney relationship until you make an appointment, we discuss your case face to face, I accept a retainer, and we explictly agree to enter into representation.