When a new friend of yours in turn befriends one of your old friends and monopolizes his/her social schedule, effectively locking you out from that old friend.
She's a total friendjacker -- Magnus can't go biking with me because they are rock climbing AGAIN!
A person who creeps on your social networking page, easing their way through your friends list and sends friend requests to them in the hopes that they will accept the invite due to you being a mutual friend.
My brother Josh is a fucking FriendJacker.......all of a sudden we have 86 mutual friends. What a douche!