I'm going to ruin my ex-girlfriends life by telling the guy she cheated on me with a secret she told me just before she cheated
Do you think he even cares?
Yes, he tried to pursue her after she broke up with me
I think that you should reconsider. She obviously told you that in confidence, you should not use that secret to hurt her down the road.
I'm going to post it to facebook anyway.
Don't you think that posting that might make her post something secretive about you? Potential partners might see what you are doing and want to avoid you if they think this is the way you act when you are in a relationship. It would be better to just move on and not act bitter.