I love this piece of marketing from Burger King: Whopper Sacrifice. You use the Facebook App to "sacrifice" 10 friends by un-friending them, and you then get a coupon for a free Whopper. Those friends receive messages stating they were unfriended by their former friend and a chance to install the app and get a burger for themselves.
People I know who are using it are dumping friends for the burger but will likely re-friend them later which is just perfect. Good stuff.