In the past decade, we have witnessed the emergence of large-scale media-sharing social network communities such as Napster, Facebook, and YouTube, in which millions of users form a dynamically changing infrastructure to share multimedia content. This proliferation of multimedia data has created a technological revolution in the entertainment and media industries, bringing new experiences to users and introducing the new concept of web-based social networking communities. The massive production and use of multimedia also pose new challenges to the scalable and reliable sharing of multimedia over large and heterogeneous networks; demand effective management of enormous amounts of unstructured media objects that users create, share, link, and reuse; and raise critical issues of protecting the intellectual property of multimedia.

In large-scale media-sharing social networks, millions of users actively interact with one another; such user dynamics not only influence each individual user but also affect the system performance. An example is peer-to-peer (P2P) file sharing systems, in which users cooperate with one another to provide an inexpensive, scalable, and robust platform for distributed data sharing. Because of the voluntary and unregulated participation nature of these systems, user cooperation cannot be guaranteed in P2P networks, and recent studies showed that many users are free riders, sharing no files at all. To provide a predictable and satisfactory level of service, it is important to analyze the impact of human factors on media-sharing social networks, and to provide important guidelines for better design of multimedia systems.