IF the threats are of a physical nature and expressly or impliedly indicate imminent harm to any of you, print them out and speak to the police. As to the "slanderous" comments (actually, slander would refer to the spoken word and libel to the written word), consult with an attorney in your locale to send a Cease and Desist letter against anything that actually constitutes defamation.
For starters you can file a complaint with facebook. You can also print out the comments and if they are truly threatening, you can take them to the police department that has jurisdiction in your area and report the threat. You may have a basis for a lawsuit for their statements, but remember even if you sue them and are successful, which is a long tortuous process, any judgment you are awarded may only be worth the paper it is printed on. If you believe you have been truly slandered", speak to a personal injury lawyer immediately. Good luck.
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