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Honestly, it's a childish thing to do. To me, if a guy has to 'announce' to the world that he is seeing someone new, then he's obviously trying to compensate for his desire to throw it in your face. Not really over you quite yet, I would assume!
I had an ex do this as well. He posted his new relationship on facebook, which he never does. He did this when he had split with an ex fiancee as well several years ago. So I knew what was up. He was upset that I left him, but he cheated on me and I didn't want to be with someone I couldn't trust. He never owned up to it, but I had lots of proof of what was going on.
Honestly, this person is probably upset that you are not speaking to them, so they do this to try and hurt you. Just act like it doesn't hurt you. Don't pay any attention to them. They don't deserve the time and space inside your head :)
Go out and improve yourself. Live your life and be happy for yourself. Surround yourself with positive people who support and love you. Don't pay any mind to this person.