Research has shown that a large number of people have used social networking sites (SNS) excessively, and this may lead to various negative consequences. To alleviate SNS addiction, this study investigates the role of SNS characteristics in the formation of SNS addiction. By applying the incentive sensitization theory in the context of SNS addiction, we suggest that compulsive motivation for using SNS is developed by pleasurable and rewarding SNS use experiences. We find that social network characteristics and communication characteristics, which determine the rewards that users obtain from SNS use, moderate the relationship between SNS use habit and SNS addiction. Novel behavioral measures of SNS use habit and SNS addiction, based on SNS activity logs, were developed. The research model was empirically tested using a large and unique dataset. Besides contributing to theoretical development on SNS addiction, the results of this study offer practical options to prevent SNS addiction. Moreover, the measures of SNS addiction make possible automated monitoring of user behavior on SNS to detect potential SNS addicts.