The insight into social networking that Burger King uncovered is so simple I can't believe it was overlooked for so long. The Whopper Sacrifice asks "what are your Facebook friends really worth?" Pure genius. BK completely flipped the Facebook value proposition on it's head by rewarding users that sacrificed 10 of their "Facebook friends" for a free Whopper through a viral application. More than 230,000 friends were removed by 82,000 users during the very short promotion.
Due to a few technical issues around privacy the promotion was pulled but I give BK (and CP+B) a lot of credit for producing an idea like this.