Many studies examine antisocial behaviours on social media-such as sharing misinformation or producing hate speech-but far fewer examine how platforms can incentivize more prosocial behaviour. We identify several ways in which social media platforms currently enable such behaviour, including (1) connecting new communities, (2) enabling collective problem-solving and (3) expanding the boundaries of philanthropy. However, we also discuss how some of the factors that enable prosocial behaviour can also empower malicious actors-as well as the challenge of creating prosocial behaviour that is sustainable and impactful offline. We then propose a research agenda to help scholars, policymakers and corporate leaders to identify the causal factors that shape prosocial behaviour on social media. This agenda focuses on (1) the size and shape of social networks, (2) platform affordances, (3) social norms and (4) how prosocial behaviour can be embedded within existing and future business models of social media.