In this study, we attempt to understand one frequently observed paradox in user social networking behavior – highly concerned about privacy issues on social networking sites, yet actively participating in social networking activities. Based on qualitative analysis of student essays on their social networking activities and perceptions, we propose a theory for user online social networking behavior – the adaptive cognition theory (ACT). The main argument of the theory is that user behavior toward social networking is dynamic and adaptive primarily influenced by the perceived benefits and risks. More often than not, the perceived benefits dominate the perceived risks in user behavior calculus, resulting in the commonly observed phenomenon that users seem to ignore privacy concerns when participating in social networking activities and using social networking web sites. We argue that ACT explains user social networking behavior better than well-established behavioral theories do such as TAM, TPB, and rational choice. Furthermore, ACT provides prescriptive insights for managers of social networking sites and companies interested in taking advantage of the social networking phenomenon. Limitations and future research directions are discussed as well.