It's a joke, but it won't be obvious to everyone. The thing is, it's a public posting on Facebook that everyone can see. If you joke about bitch-slapping someone or shooting someone you don't like in private, that's understood as a joke by those familiar with the situation. If you do it in public, that can be counted as a grave threat.

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C over-reacted, but has a point. If you want to make crude jokes, change the privacy settings on the post. There are children and animal rights activists everywhere on Facebook... COPPA (13 yr old below restriction for personal data gathering on US websites) be damned... this ain't the USA.