This study evaluates the relationship between social media usage and state sad and anxious mood in a sample of young adults (N = 670) in the years 2019 through 2021 using cross-sectional, experimental, and ecological momentary assessment approaches. In our cross-sectional data, objectively measured usage of Instagram and Twitter was not associated with state sad or anxious mood after adjusting for multiple comparisons. We used an experimental design to test the effect of low versus high acute social media usage on state mood and did not find an effect of time spent using social media on state sad or anxious mood. In our ecological momentary assessment sample, participants reported their time spent on social media and state sad and anxious mood six times per day over 7 days, and we found a weak, positive, bidirectional relationship between social media usage and state sad and anxious mood. In this study, evidence for a relationship between social media usage and sad and anxious mood was inconsistent, and effects were small in magnitude. Time spent on social media sites may be less important for mood than other aspects of social media usage, such as the type of content accessed.