Ex-husband slanders wifes name on Facebook?

I recently got officially divorced from my exhusband about 4 months ago but have been seperated for 6 months. I started dating a mutual friend facebook official about a week ago. but we have been hanging out for a couple months and he has been my good friend for a couple years. My ex husband finds out about this and writes a nasty note about him and I and tags my father and sister in it among his friends and family. And completely slanders my name for the world to see. What do you think about this? What is your opinion?