PeerWise, an online tool that facilitates peer learning through student-generated content in the form of multiple-choice questions, was implemented in a large introductory university physics course. Interactions between students engaged in PeerWise were investigated using social network analysis. This showed that a dense and relatively equitable network was formed, giving students direct and easy access to the whole cohort through sharing and answering questions. A statistically significant correlation was found between students' use of PeerWise and their performance in the end-of-course examination, even after taking into account their prior ability as measured by assessment prior to the start of the course. This suggests that students benefit from engaging with their peers, not only by sharing or answering a large number of questions but perhaps by being exposed to a wide range of question styles, levels, explanations and comments.