Social media use occupies a prominent space in social sciences scholarship and beyond. However, the distinction between active and passive use of social media, although important in explaining a variety of users’ behaviours, has been overlooked in terms of its potential to predict key socially relevant outcomes like beliefs in conspiracy theories. In three studies (N = 1388, in total), we provide evidence on (a) the role of passive social media use in believing in conspiracy theories via personal relative deprivation; (b) the interaction effect between social media use and personal relative deprivation on beliefs in conspiracy theories. The results showed that passive social media use is linked to, and increases, beliefs in conspiracy theories and this relationship is partially mediated via increased personal relative deprivation (Studies 1 and 2). In Study 3, personal relative deprivation was also manipulated experimentally in the context of a ʻmoderation‐of‐process' design, yielding a significant interaction with social media use. The results are discussed in the light of their social and media psychological contribution and implications in the digital era, when misinformation often becomes prevalent.