It's only defamation if it isn't true. Unfortunately, because you posted it online, it's still considered public. I would recommend talking to your HR person. If the content of the objectionable post doesn't or wouldn't actually affect your ability to do your job, or isn't related to your job, your argument will be stronger. Furthermore, if there is bad blood between yourself and this person (I'm assuming there is), that should also be taken into consideration. I would also investigate if your company has any sort of social media policy that governs behavior like this.