Discuss it with a lawyer. I suspect you will be told that you are contributing to your problem by allowing it to stay up.
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You might have a case, but how are you going to prove you were not hired because of the negative post on your FB/Twitter? Just in case, make a copy of the negative post then delete it. No sense in continuing to allow others to see her negative comments. You will have a hard time getting any evidence why you didn't get the job from those processing your job applications. No one "wants to get involved" in being a witness in a lawsuit.
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