Social technologies including blogs, wikis, social bookmarking sites, photo sharing, video sharing and social networking sites (SNS) have been widely used to facilitate online social networking (OSN). We define OSN as a range of activities enabled by online social technologies and operationalised by a group of people. OSN is widely popular for non-educational purposes among students. However, OSN also has the potential to be appropriated and repurposed to support teaching and learning delivery in a formal learning environment. Despite the availability of implementation cases and trials, detailed studies on why and how lecturers and students appropriate and repurpose social technologies for OSN in education are still lacking. In addition, these understandings are seldom guided by any theoretical frameworks. This paper suggests the use of Activity Theory as the theoretical lens in investigating the use of OSN in higher education. A conceptual model of how social technologies can be appropriated and repurposed guided by the theoretical understandings is proposed and discussed.