Social networking sites such as MySpace and Facebook thrive on energetic social interaction, but the factors that assure this are not well understood. There is a lack of theory that can describe and predict the successful adoption of new social computing systems. This paper introduces the Social Software Performance Model, and uses it to interpret the evolution and usage of social networking sites. Drawing from socio-technical systems theory, task technology fit, and structuration theory, this model identifies the components of social software, and describes their role in the evaluation and adoption of these systems. The results of three studies are presented, providing initial empirical evidence for the model.