Facebook announced their developer platform about a year ago in September of 2007. It generated a lot of interest in the developer community. A lot of applications were created and developed in the subsequent weeks/months.

These very first applications were basically simple implementations of some of the existing behavior/features that existed in facebook. A few that come to my mind are SuperPoke, SuperWall, FunWall, Send Hotness etc ...

These applications became rapidly popular in part due to the fact that these were something new for the users to play with and also the inherent viral nature of the social network. A user could potentially invite all his friends to use the application and so on.

The application needs to have a social nature built into it. It being part of a social networking site, it's really easy to tap the social graph and link it to the application.You have to make sure that a user can relate to the content and similarly have his friends from the social graph relate to it.

I like to use' Friends for Sale' as an example. In this application you buy your friends and then it sends a notification to the friend that he/she has been bought by the user. The user can then login to the application and see their worth. He can in turn buy other friends or even the pets of his owners provided he has sufficient virtual currency.

You should design your application around a virtual economy. It will definitely help in monetizing your application once it gets viral. It's a lot harder to come back and add the virtual economyto an app.

The Virtual economy will allow the users to earn virtual currency that could later be spent on virtual goods. The currency could virtually be used to puchase anything that would enhance the user's experience of the application.

The user can earn the virtual currency in numerbale ways. A few method is to give the user an incentive for using your application on a frequent basis, thereby increasing the CPM value of the page. Other methods include, using actual dollars to buy the currency or giving the users the option to complete surveys in exchnage for the virtual currency.

It will also help if you can have some sort of a competitive nature built into it. The more the competition, the more addictive the application becomes. You need to expose the human nature of being competiting and be the best.

This could be tracked by either a leaderboard or the worth of a users virtual currency. The more power/previliges a user enjoys would give him an edge over his other friends and constantly keep the users on the edge to outdo the others.

In 'Friends for Sale', the more money you have the more number of friends you can own as pets. On the other hand, there is an application called 'Green Path', where you indirecly comepte to save trees.

It's a good idea to have a community built around the application. It helps a lot to have forums, walls, reviews built into the application. The more activity that application users perform, the close knit the commnity gets. This creates a sense of belonging to the users where they can interact with other fellow users.

This is one of the critical aspects of having a succesful applications on the facebook platorm. You have to make sure that you listen to the users and steer your development based on the feedback. The mantra should be to give the users what they want. If you can keep them happ, they will come back to you again and again.