10 Commandments for Facebook Applications... if you want a successful application. If you don’t want a successful facebook application; ignore at your own peril.
1. Make it simple. Users DO NOT read.
2. Don’t give users more than 2 choices for navigation on a page
3. Make invites part of the process for using the application initially
4. Use viral carriers: Mini-Feed, Notifications, invites, Messages, etc.
5. Give users a reason to come back
6. Let the application be the viral carrier
7. Focus on User Engagement
8. Start with Limited functionality
9. Always measure: adds, deletes, blocks, invites, page views, length of visit, etc.
10. Never stop iterating and changing your app.
There are another 20 commandments for facebook applications. But this should keep you on the path initially.