Not only can you use Facebook applications to market your own site or service you can also generate revenue from your application.
You can place code in your application that allows others to purchase clicks. You then make a portion of the price an advertiser paid.
Some work on a Cost Per Click model (CPC) for the click through in your application. For example, if an Advertiser pays 10 cents per click and you receive a click at 50% you would receive 5 cents. Others work on a Cost Per Thousand (CPM) model in which you receive a set price for every 1,000 advertisements shown.
These three advertising networks focus exclusively on Facebook and work like any other ad network, just copy and paste the code in your application.
fbExchange is an example of an advertising network that pays you on a CPC model. You simply add their code to your application and you are generating revenue.
Cubics is an example of an advertising network that pays you on a CPM model, a set price for every 1,000 advertisements displayed. As of the writing of this article they are paying 33 to 81 cents per 1,000 views.
Appsaholic is part of the Social Media network and is not currently accepting publishers as of the writing of this article. However, you may buy advertising on a CPC model.
How large the advertising network on Facebook is remains to be seen.