For the first few months of Facebook's Platform, the company ranked applications based on the number of users who had installed the application. The new metrics give a number of daily active users, along with a percentage that seems to represent the daily active users in comparison with the installed user base. The change cut numbers dramatically. Super Wall, for example, was previously listed as having more than eight million total users, and now it shows up with only 800,000 daily active users. Meanwhile, extremely simple games such as Zombies and Pirates vs. Ninjas were forced to adapt to the metrics switch by adding functionality to their existing viral design.

"We were primarily focused on overall user numbers, originally," says RockYou's Shen. But he says that moving to the new model has been good for the company's products because it has forced him and his colleagues to focus on making their products more interesting to use, instead of just easy to share.

But while the metrics change makes sense on the surface, Farmer says, "nobody except Facebook really knows what those numbers mean." He says that he hasn't been able to determine what qualifies a user as active. For example, he says, when someone posts a photo to Super Wall, it's not clear whether that is being counted as a single action or as an action on the part of every person who receives the post.

Regardless, Super Wall is a fiercely viral application. "Whenever we're going to develop new functionality into an application, we think about what you're going to do as a specific person, and how that triggers actions from all of your friends," Shen says. When a user posts a drawing to Super Wall, he says, the application makes it very easy to share that post with all the user's friends. This aggressive strategy to keep users engaged seems to be keeping developers like RockYou and Slide on top for now. The developers make money by including ads in their applications and by helping other developers make their applications more viral, for a fee. For example, Yahoo recently enlisted RockYou to help market its struggling music-video application.

B.J. Fogg, who directs research at Stanford University's Persuasive Technology Lab and will be coteaching a class on Facebook applications, says that the fast pace of Facebook application developments suggests a new model for how interactive systems will be made in the future. "I think one of the things to explore is consumers writing their own interactive applications that they would then share with their social network," Fogg says. "Facebook users wrote code to answer their own needs--Extended Info and Top Friends and Super Poke--and created stuff for themselves. And that might be the model of how interactive experiences are created in the future."