Abstract
This chapter describes the mechanisms of presence in social networks and presents an ontology that frames the purpose, content, methods of production and methods of consuming presence information. The concept of presence in social networks has been steadily evolving along with the Internet. Recognised as an essential feature of all instant messaging services from the IRC onwards, mechanisms for constructing and consuming presence information have become more elaborate, with the addition of more sophisticated mechanisms for producing, consuming and representing presence. A model for systems that offer presence services is developed, and this enables a number of future trends to be identified.