Being fucked on Facebook by a fake account; fake account meaning that someone else, usually a fat fob, is behind the account (using stolen pictures of someone else, who is usually attractive).
The term originated from Haroon Khan, a fat kid who was unable to get the time of day from a girl named Malaika whom he met over Facebook and made him horny. He then went on to make another account with the picture of someone else, and started having Facebook sex with her, ie. Jacking off while having dirty conversations in Facebook chat.
You just got haroon'd, Malaika!
That muscular guy you were talking dirty with over Facebook wasn't who you thought it was ... you got haroon'd, Eman!