This study examined the usefulness of a mobile instant messaging (MIM) tool to support teaching and learning. Taking a usefulness theoretical perspective, we examined in detail the utility and usability of using WeChat in a postgraduate-level course. Multiple types of data were collected and analysed, including MIM interaction records, questionnaire surveys and semi-structured interviews. The results show that, overall, MIM can be utilised in a variety of ways to support teaching and learning. These include promoting transmissive, demonstrative, dialogic, journaling, helpline and notebook activities. We also found MIM to be particularly suitable for promoting affective social presence (expression of emotions, selfdisclosure) and cohesive social presence (phatics, vocative). Four specific technological attributes of MIM that facilitated information sharing among students were reported. Implications for practice and research, including challenges and possible solutions, are proposed, followed by a discussion of the study's limitations and suggestions for future research.