Social network service (SNS) has become a popular media for social interaction and information exchange. However, it is not clear why people use SNS. In addition, human behaviour researches related to SNS are not sufficient despite of its prominence. Hence, the goal of this paper is to explain why people use SNS. First, we empirically examined what individual’s characteristics affect the use of SNS. To examine these characteristics, we developed a structural model on the basis of three popular theories; Technology Acceptance Model (TAM), Network Externality, and Innovation Diffusion Theory (IDT). Also, to achieve our goal, we applied Structural Equation Modeling (SEM) approach to evaluate the effect of each construct. The finding shows that Perceived Usefulness (PU), Perceived Ease of Use (PEU), Members (M), and Compatibility (C) have a positive significant effect on the Actual Use (AU) of SNS. Moreover, we identified the differences between Facebook users and Twitter users. From the control variable (Facebook and Twitter users) analysis, we concluded that PU and M are important factors to Facebook users, while PEU and C are prominent to Twitter users.