This paper reports a study that investigated the social interaction pattern of collaborative learning and the factors affecting the effectiveness of collaborative learning in a social networking environment (SNE). A class of 55 undergraduate students enrolled in an elective course at a Chinese university was recruited for the study. The participants used an SNE to support their learning activities over a semester. Data was collected through interviews, classroom observations, and from digital artifacts created in the SNE. Social network analysis and content analysis were adopted to analyze data. The results showed that social interaction in the SNE tended to be teacher-centered, social-oriented, fragmented, and short-lived. Pre-existing social relations, perceptions of the teacher and peer feedback, preferred learning approaches and collaboration strategies, nature of the task, and the individualistic nature of social networking technology were factors that influenced how students interacted with each other in collaborative learning mediated by the SNE. The findings have both practical and theoretical implications.