This comparative case study examined factors affecting behaviour and learning in social networking sites (SNS). The behaviour and learning of two classes completing identical SNS based modules of work was observed and compared. All student contributions to the SNS were analysed, with the cognitive process dimension of the Revised Bloom's Taxonomy used to measure the type of thinking that students demonstrated. Key findings include the trade off between social and learning contributions, the potential of SNSs to enhance motivation and digital literacy development, and the critical role of the teaching in influencing the behaviour and learning that transpired. Effective teacher implementation in the SNS was associated with positive teacher-student relationships, establishing a "learning" rather than "social" attitude towards the SNS, and the online presence that the teacher exerted. (Contains 7 figures and 3 tables.)