You have to understand the fundamental concept behind FaceBook and other social networks to begin using it's ad platform for a substantial ROI. Unlike Google, Bing and Yahoo! it's not driven by search terms that a user inputs.
With search engines, people are directly looking for a solution to a problem, for example, if someone was to type in "lose weight" most likely they want to lose weight, or if they search "learn golf", indeed they probably want to learn golf. It's quite easy to make a direct sale based on someones search query, you simply offer them a solution to their problem.
On the other hand, targeting methods on Facebook are very different. You're not trying to provide a direct solution to a problem...
Facebook, as many have stated above, is driven on social aspects such as chatting, communities etc, hence the reason why social games work so well on the Facebook platform, in fact if I remember correctly, Zynga (publishers of Mafia Wars, Farmville etc) posted a revenue of $1B for 2009.
If you're goal is to directly sell your target a product, then Facebook isn't a good option for you. However, if you've got the money to spend on brand and relationship building, then indeed you may be able to make Facebook worthwhile.
Forgive me if this post sounds stupid, I could usually articulate this in a much better way, but it's 00:17 and I'm as tired as fuck.