With 700 million+ users, Facebook has the audience your app needs. Monetizing your Facebook app is as important as building it in the first place. Facebook offers two internal ways of making money such as Facebook Ads and Facebook credits. These tools can help the business sell their products, market other products and do affiliate marketing.
With Facebook ads, you can promote your product, affiliate products, build an email list, and generate leads through Facebook Ads.
For more information: http://www.facebook.com/advertising/
With Facebook credits, you can charge a user for providing them a service through Facebook.
For more information: http://www.facebook.com/credits/
Third Party Vendors such as RockYou
Use a third party service that setups up the ads for you and takes care of the marketing.