The act of removing a friend from your
facebook account.
Compulsive people prune their friend list periodically, removing people that they no longer have contact with. More often though, unfriending is only done when a particular friend's updates and self-promotions become so
annoying that you can no longer stand hearing about them. Or you might unfriend someone when they piss you off, however, this is not very effective since the person who is unfriended is not notified that you unfriended them and you'd be better off to keep them
as a friend and plot your revenge.