Most studies on traditional asynchronous online discussion suggest that facilitating dialogue, that is, commenting forum activities, result in better learning and performance. However, due to open entry and diverse learner backgrounds, learner behaviour in massive open online courses (MOOCs) may be different. Viewing forum messages, which involves fewer mental and physical actions as well as less cognitive processing, rather than posting forum massages, might better match the learner's study purpose. In this study, we investigated the effects of three common types of online MOOC discussion forum activities (viewing, voting and commenting) on student peer learning (peer reviews) and performance (quiz scores). We used stepwise regression models to analyse two data samples of a humanity and art MOOC provided by a private university in the United States for exploring factors influencing peer learning and performance. The results indicate that peer learning and performance were primarily predicted by viewing, and to a lesser extent by commenting. The three plausible explanations for the findings are the learner's study purpose, weaker instructor-learner ties, and voluntary forum participation. We suggest ways to encourage viewing messages in forums and present limitations and further directions.