The promise of social network technology for learning purposes has been heavily debated, with proponents highlighting its transformative and opponents its distracting potential. However, little is known about the actual, everyday use of ubiquitous social network sites for learning and study purposes in secondary schools. In the present work, we present findings from two survey studies on representative samples of Israeli, Hebrew-speaking teenagers (N1 = 206 and N2 = 515) which explored the scope, characteristics and reasons behind such activities. Study 1 shows that these can be described best as online knowledge sharing, that is: the up- and downloading of knowledge and knowledge sources to social network-based peer groups. Findings were replicated in study 2 to further support the claim that school-related knowledge sharing is common and widespread and entails different types of knowledge. Findings from study 2 furthermore show that sharing is mainly motivated by prosocial motives, as well as expectations for future reciprocation. Sharing is predicted by individual differences, such as gender, collectivist values, mastery goal orientations and academic self-efficacy. Relations between competitive-individualist values and sharing are more complex, and are, among others, moderated by expectations for future benefits. Implications for educational practices and for learning are discussed.