In recent years, online social networks (Online Social Networks) have dramatically expanded in popularity around the world. According to the data in October 2012, Facebook has 1.01 billion people using the site each month. Moreover, the numbers of users in five popular Online Social Networks are listed in Table 1. The rapid growth of Online Social Networks 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 Online Social Networks