Social networking services (SNSs) have recently emerged as a research topic of interest, in line with their commercial success and popularity. They are internet (sometimes mobile) services that have, as a primary purpose, the building and sustaining of users’ social networks. In this article we conduct two analyses. We review the existing literature on social networking software, and we examine the functionality of four leading social networking services: Facebook, MySpace, Second Life, and Twitter. The two analyses are iteratively matched to provide an initial account of six characteristics evident both the in services themselves, and the literature which discusses them. The characteristics help shape the area of study, and can be tested and developed by more rigorous forms of research.