Our survey contains four aspects of understanding user behavior in Online Social Networks. First, a social graph is a classic and effective mathematical model to represent the relationship between users in Online Social Networks, and has been widely used in Online Social Networks research. Based on four different types of social graphs, we discuss the aspect of connectivity and interaction. Second, network monitoring records detailed traffic activity of Online Social Networks and provides us with a method to understand the network usage of Online Social Networks. Also, network-based measurement results can demonstrate more users’ activities than using the social graph only. Therefore, we focus on the perspective of traffic activity. Third, the rapid development of mobile platforms and applications plays an important role in Online Social Networks related applications. Mobile devices not only provide a venue for users to access Online Social Networks everywhere, but also establish mobile-centric functions like location-based service (LBS) in Online Social Networks. Understanding the behavior of mobile users can be leveraged to enhance the performance of mobile social applications and systems. Thus, we review the studies of mobile social behavior. Last but not least, Online Social Networks introduce new challenges related to security and privacy. Malicious behaviors, such as spam and Sybil attacks, take place in Online Social Networks and bring severe security threats. We show studies of malicious behavior.