The integration of social media in formal education has received consistent interest in the recent decade. While most studies have focused on the mandatory use of social media as pedagogical tools, less explored is the context in which students voluntarily choose and use social media to supplement their formal learning. In this study, we employ an affordance approach to exploring the resources in social media that students perceive and use voluntarily for formal learning. Drawing on a qualitative study using focus groups of university students, we identified ten categories of social media affordances for learning in the voluntary context: association, editability, immediacy, interactivity, manageability, modality, persistence, scalability, searchability, and visibility. The affordances mentioned most frequently by students were visibility, persistence, and interactivity. These identified affordances encompass the major resources offered by social media for learning in the voluntary context, and provide insights into understanding, evaluating, and guiding social media adoption in formal education as well as lifelong learning. This paper concludes by presenting limitations and implications for research and practice.