You may be about to become an unpaid spokesperson on Facebook
If you're on Facebook, that is. Facebook has rolled out a new feature called Sponsored Stories. If a brand name comes up in your activity, say you check in at Starbucks or you say you "like" Starbucks, then Starbucks can buy that update and turn it into an ad. Who do they show that ad to? All your friends. No warning and you can't opt out. Your picture shows up too.