We examine third-party Online Social Network (OSN) applications for two major OSNs: Facebook and RenRen. These third-party applications typically gather, from the OSN, user personal information. We develop a measurement platform to study the interaction between OSN applications and fourth parties. We use this platform to study the behavior of 997 Facebook applications and 377 RenRen applications. We find that the Facebook and RenRen applications interact with hundreds of different fourth-party tracking entities. More worrisome, 22\% of Facebook applications and 69\% of RenRen applications provide users' personal information to one or more fourth-party tracking entities.