Facebook now supports cost-per-action pricing for ads through API

This pricing model lets advertisers define the action that is most important to them and set a maximum price they’re willing to spend for each action of that type. For example, instead of paying for every click — whether it came from a user who simply viewed a photo larger, left a comment or visited the advertiser’s fan page instead of actually Liking the page — a company could now say it only wants to pay for page Likes.