I had the same conversation yesterday with my 18 year old. I told her that when people come at you with defensiveness, fabrications, hostility, whatever...they are usually projecting something on you. Who cares what it may be, though, so don't spend too much time wondering how you became the target.
Mean people are all around us. The trick is to know when to make it an issue and when to shrug your shoulders and walk away.
As far as I am concerned, unless its a direct threat or slander, best to walk away.
And ditto on Facebook being evil. It is a safe place for people to write things they would never say in person.