The rise of social media has dramatically altered the social world – introducing new social behaviours which can satisfy our social needs. However, it is yet unknown whether human social learning strategies, which are well-adapted to the offline world we developed in, operate as effectively within this new social environment. Here, we describe how the computational framework of Reinforcement Learning can help us to precisely frame this problem, diagnose where behaviour-environment mismatches emerge, and develop novel solutions to realign our behaviour with the new social media environment. The Reinforcement Learning algorithm, which has proven successful in characterising empirical aspects of human social behaviour, consists of three stages: updating expectations, integrating subjective costs such as effort, and finally selecting an action. We suggest that specific social media affordances might interact with parameter-based computations at each of these stages, in some cases exploiting Reinforcement Learning biases by violating the environmental conditions under which these strategies are optimal. Characterising the impact of specific aspects of social media through this lens allows us to understand how digital environments shape behaviour and in turn impact outcomes such as mental health. Further, human Reinforcement Learning could inspire both user-based interventions (such as cognitive therapy) and environment-based interventions (such as platform design and policy regulation), to improve the mental health outcomes of social media use.