I don't think most people make money actually ON facebook. It's more a marketing tool as it has such a high concentration of users.
Example:
http://apps.facebook.com/rss-feed-reader
That app I give out free. It has 420 installs + right now. Problem is my RSS isn't updating 100% so I only have around 50 active users at any time. It's a cool tool to add your blog but has "Powered by Bloggeries" at the bottom and you can click through to bloggeries. Since everyone installing it is a blogger chances are those coming through are targeted traffic. That's why I had the app built.
I've heard that paid advertising on facebook isn't so hot.
You may wish to read this:
Web 2.0 Expo - Facebook Marketing Best Practices | Online Marketing Blog
I hope this helps,