The service http://www.paywithatweet.com/ lets you sell digital goods for tweets. Before giving you a download link for some digital product (a song, a book or software), it asks you to log in with Twitter or Facebook and posts a message there along the lines of “Hey! Check out this new something!”.
I think it could be used to buy physical goods too. Bolted onto the cart of an ordinary online store, for example. At checkout, when the user sees the final amount to pay, he could use his social network like an ordinary discount coupon: tweet or post a message on Facebook, and the amount to pay automatically drops by a certain number of monetary units.
Commercializing Twitter and Facebook? Selling out your friends for pennies? Fuck yeah!!!