1. Target the friends list
One of the biggest draws of developing an application for Facebook is the unprecedented access to the social networking site's massive user base -- its so-called "social graph." Developers can reach a broad audience with very little effort.
"Typically, software developers struggle to get anyone to use their products, regardless of how useful," says Haroon Mokhtarzada, CEO of Freewebs, which created the medieval fantasy game Warbook (requires Facebook registration to view) for Facebook.
Warbook invites you to "Control a kingdom, build an army and conquer your friends!" (Click image to see larger view.)
So the developers of the most successful applications say to exploit the social "interconnectedness" of the Facebook user base. It can be used to target the demographic you want to "sell" your application to.
"Facebook doesn't give you all the different social graphs of data that it has," Commagere explains. "The friends list is the most readily accessible one, but many others can be calculated. I could construct a query against Facebook servers that determines 'friends that love movies.' I can do things with the results of that query, like encourage [the user] to interact with these friends through an app that features movie-related stuff, like 'Send a movie quiz to these three friends because they love movies like you!' What you can typically query is always a subset of the friends list."