Inside Facebook reports that Facebook has added new metrics for app developers to track the spread of their application. The new "Application Adds by Facebook Referrer" displays how many application ads were a result of the app directory, the profile box, the news feed, the mini feed, Facebook search, or requests.
These stats will certainly be helpful for app developers to gauge the best way to market and promote their apps, and whether their marketing campaigns on Facebook ad networks like Social Media are worth their time and money. But just how most Facebook apps spread is likely to remain hazy until more data can be gathered from multiple app developers.
Justin Smith has data from a friend's application, which shows that the profile box is far and away the most popular channel for attracting new users to an application. Surprisingly, given the amount of press about how ingenious the news feed and mini feed are for spreading things virally, they do not factor nearly as much into the overall spread of this particular application. That said, Inside Facebook's example is a very small sample size for one (undisclosed) application. As Smith notes in his post, "the relative importance of viral channels will depend on your application."
The Zombies app probably benefits more from requests, for example, because of the way it encourages people to invite users, while the Where I've Been app probably gets many installs from the profile box, and the Movies app likely does well in the product directory as a result of its prominence there.
Would any app developers out there care to share some of their data? It would be great if Facebook would release data across the entire network in aggregate, but until that happens, if you want to share trends you've notice for your application. please do so in the comments.