I remember a Coca-Cola & 7/11 promo about a year back, where if you "Liked" the 7/11 page on Facebook they would give you a printable coupon for a free Coke. The first problem was that the barcodes on the coupons didn't scan so well after being printed out on a home printer and crumpled up in somebody's pocket for a few days (which undoubtedly cost 7/11 thousands or tens of thousands of man hours) and the second was that you could simply refresh the coupon page to get /as many coupons as you liked/. Needless to say, the promotion ended not soon after.