Social media has been widely used as an important source of health information, particularly during public health crises. However, findings regarding social media's impact on young adults' mental health are mixed. There is a need to identify social mechanisms underlying the effect of social media on mental health outcomes. Our study breaks new ground by proposing and testing a moderated mediation pathway from social media use to stress in young adults during the COVID-19 pandemic. We conducted a two-wave online panel survey in the context of China. With a general basis of the Street's three-stage model, our results indicated that social media failed to directly affect young adults' stress. Instead, fatalism completely mediated this relationship. Also, one's perceived social media exhaustion negatively moderated this mediation pathway. The findings have significant theoretical and practical implications for the use of social media to promote health, well-being, and quality of life among young people during public health crises.