Research and practice in the use of social networks for educational purposes have indicated the potential of Facebook as an online supplementary learning platform. Yet, empirical evidence regarding students' perception and acceptance of Facebook remains limited. This study adopts an exploratory sequential mixed methods design to understand factors that affect students' intention to use Facebook as a course-based online supplementary learning platform and the mechanisms through which these effects are exerted. Interview data with 11 students and questionnaire responses from 407 students suggested that perceived enjoyment plays a salient role in influencing students' behavioural intention by mediating the effect of pedagogical and social affordances of Facebook as well as perceived usefulness and perceived ease of use. On the other hand, perceived usefulness had no significant effect on students' behavioural intention. The mixed methods design and the findings reported in the study offer useful implications for researchers, teachers, and students in promoting the use of Facebook as a supplementary learning platform.