Researchers and practitioners alike recognise the importance of emergency management (EM) in limiting the adverse impacts of crisis events, as well as the promise of social media to support these efforts. Decision making, which is crucial to ensure the effective management of immediate, emerging, and sustained crises, is one facet of EM potentially affected by social media. While much research has investigated social media in a crisis context more generally, little is known thus far about what it means for EM decision making. In this paper, we investigate the current knowledge base of this phenomenon and infer from it factors that are crucial for its understanding. To this end, we propose an analytical framework of EM decision making based on previous work on complex problem solving and social media networks. We then systematically review and rethink existing research from a decision-centred point of view to identify and synthesise key findings that are relevant to the role of social media in the EM decision-making process. Finally, we outline the research gaps that need to be closed to arrive at a more comprehensive understanding of social media for EM decision support and to begin moving towards theoretically grounded explanations of the phenomenon.