Greta Thunberg is a major figure in climate change discourse, featuring prominently in the news and social media. Many studies analyzed the “Greta effect,” but there has not been a review that integrates the diverse literature and consolidates insights into her impact on social media. We provide such a synthesis with a narrative review of 63 peer-reviewed publications that gauged social media reactions to her from different disciplinary perspectives, with various methods, and across a range of contexts. We show how social media have both helped Thunberg mobilize her supporters and harbored backlash from her enemies. This twofold effect varies across different contexts. Yet a comprehensive assessment of the “Greta effect” remains elusive since Thunberg borrows from different kinds of activism but does not neatly fit into any of them. Our review shows how social media have become the most important terrain for contestation around climate change.