On-line Social Networks (OSN) have become the most popular Internet service today. OSN are being embraced by companies and organisations to help connecting people, assist dealing with cooperative tasks, and develop marketing and public relations campaigns. Despite all their benefits and advantages, as happens with every new technology, they are prone to several security issues. In addition to privacy concerns, there are many other dangerous vulnerabilities that affect security. In this paper, we present our Threat Modelling in OSN, which focuses on identifying attacks against users of OSN and possible countermeasures to mitigate the risks.