In recent years, online social networks (OSNs)have dramatically expanded in popularity around the world. According to the data in October2012, Facebook has 1.01 billion people using the site each month. Moreover, the numbers of users in five popular OSNs are listed in Table 1. The rapid growth of OSNs has attracted a large number of researchers to explore and study this popular, ubiquitous, and large-scale service. In this article, we focus on understanding user behavior in OSNs.