I see some big companies are marketing via Facebook:

But the trick used by burger king is totally different and the most effective campaign used ever. Fast-food chain Burger King has created "Whopper Sacrifice," a Facebook app that will give you a coupon for a free hamburger if you delete 10 people from your friends list.
Source: Sauron's Eye on Internet: Delete 10 friends from FaceBook, Get a free Hamburger

Would you ditch 10 friends for a burger? As for me (now that I am older and wiser!) I wouldn't even trade money for such a burger as I'm too scared about what they put into them...

At the end of the day though, Burger King is being creative with their marketing. Many people are talking about Burger King via their Facebook profiles (while ditching a friend here or there).

According to the Whopper Sacrifice website, currently over 216,000 friends have been sacrificed.