With over half a billion users, Online Social Networks (OSNs) are the major new applications on the Internet. Little information is available on the network impact of OSNs, although there is every expectation that the volume and diversity of traffic due to OSNs is set to explode. In this paper, we examine the specific role played by a key component of OSNs: the extremely popular and widespread set of third-party applications on some of the most popular OSNs. With over 81,000 third-party applications on Facebook alone, their impact is hard to predict and even harder to study. We have developed and launched a number of Facebook applications, all of which are among the most popular applications on Facebook in active use by several million users monthly. Through our applications, we are able to gather, analyze, correlate, and report their workload characteristics and performance from the perspective of the application servers. Coupled with PlanetLab experiments, where active probes are ...