Social network sites (SNS) are becoming increasingly important, both for individuals and organizations. These systems have affected social and cultural activities, work practices, and in particular the ways in which we discover, share and consume information goods. The functionality of SNS is emergent, shaped by user appropriation choices. In this paper, affordances are proposed as a way to understand the potential uses and future evolution of SNS. Affordances describe the characteristics of an interactive system which suggests how the system should be used. The objective of this study is to explore the affordances of SNS. The study comprises an inventory of the affordances of three popular SNS. The study reveals a diverse collection of software features which afford user behaviour in six areas of activity: social connectivity, social interactivity, profile management, content discovery, content sharing and content aggregation. The findings of the study provide a rich foundation for future research on user appropriation of SNS, the future evolution of SNS, and the design of SNS systems.