I was raped and he was never charged. Can I be sued?

I was raped back in 2006. As most people know, you never get over it. Because my rape kit was lost between the hospital and the sheriff dept (how convenient!), and because they questioned him over the phone and never mentioned the gun, he was never charged. I also know through someone who knows many people that I am by far NOT the only one he's done this to, but I am the only one who went to the police (he tried in many cases, succeeded in about half, only used a gun with me and two others from what I heard). He also gave me and the others several (curable, thank GOD!!) STD's. Fast forward 4 1/2 years. I posted something on my facebook wall not too long ago. His new wife sent me a nasty email saying he was going to sue me for slander and defamation of character. Can I be sued? All I did was let some of my friends know who he was and to watch out for him.