This plugin for FoxyCart gives your store the ability to grant discounts to customers after they’ve posted about your store or company on their facebook wall. The plugin verifies that the user has actually posted your content to their wall before the discount is granted. You can even provide tiered discounts based on the number of friends the customer has. Reward your customers with bigger discounts if they’re able to reach a larger audience for you!

See http://www.underthebedstudios.com/blog/foxycart-facebook-plugin-tutorial-foxybook/ for details.