To delete people from your friend list.
I have 2156 "friends" but only actually talk to half of them. It must be time for a Facebook Flush.
When you realize that you don't really have 5,128 friends, therefore deleting several to thin out the herd.
I hate Farmville, Mafia Wars and don't have any plans on starting a fake fish tank, so I did a Facebook Flush so I don't have to see my wall cluttered with that crap!