Daily updates on changes in user base for various Facebook apps on adonomics.com.
Met up with a developer recently whose app is in the top 20. Man, this is the biggest gold rush since 1998, and there was not a single ad agency rep at the most recent Facebook developer event. Dave McClure summed up the importance of Facebook quite succinctly: "People see shit other people are doing in the Feed, and then they click on that shit."
See the Facebook developer wiki.