My system allows restaurants to set up accounts on Nift and to create coupon offers. They can make a placard to put on their tables with a QR code to the offer, saying "Like your meal? Here's a coupon you can share with your friends, and when you share it, you get the coupon too." Nift uses the Facebook Send API to let users connect with Facebook on Nift and send links to their particular coupon to their Facebook friends.
The main inspiration for Nift are all those conversations I've had with friends, where they recommend a good place to eat, and then I never go because I've either forgotten or there's just nothing to drag me to that location when I'm planning to go out to eat. Instead, it seems like we default to whatever looks good on Yelp, which leads people to only trying the "established" choices and avoiding the variety of smaller but more interesting eating spots.
By coupling a social recommendation with something tangible like a coupon, the people hearing about this restaurant are more likely to actually go check it out.