how do you report someone on facebook for defaming someone on their facebook page?

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[Music of the Night]

Go onto their page or their post in your newsfeed. There is a little symbol that looks like a minus sign in a circle. Click it. It marks the post as spam. That hides the post and you can then undo that or report the post. Click 'it's harassing a friend' as your reason for reporting it or use whichever seems most appropriate.