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.
Dude, did you fucking unfriend me on facebook? I was looking for your email address and you're not in my friend list anymore. WTF?