Marking is the act of spamming a person's Facebook wall or status with embarrassing and outrageous statements before they have a chance to defend themselves. Properly Marking someone requires at least two or more participants in order to avoid looking like a stalker.
Ben: I hear Fred hasn't been checking his Facebook lately, we should totally go Mark him.
Mike: That's a swell idea, I will join you so that you do not have the appearance of being a stalker.
Ben: Consider him Marked