Stalking - what are my options

My narc (at least thats what i think he is) is stalking me on social networking sites. He has made public threats to me on there (no names, but he mentions things only he and I know).

So now I am not sure what to do. If i ignore him, he gets frantic and his threats escalate. Should i just appease him by acknowledging him? I almost feel like if i can get on his good side i can keep him calm. This is our only contact and I am afraid if I block him I will create even more problems. He has threatened my job and my new relationship.

What should I do?