Social media and more generally online social networking technologies have emerged as powerful tools to exchange information among a large variety of players, including the public, authorities, companies and journalists. In this paper, we review the present and potential uses of social media and how to value information they contain to manage security and safety matters. We present some examples of their uses during emergencies and crises, how relevant information can be extracted to support fighting against cybercrime and how security forces and emergency managers may benefit from integrating social media into their organisations. Finally we propose an overview of technical limitations and possible misuses of social media.