Social media usage is on the rise, with the majority of American adults using Facebook. The present study examined how Facebook activity affects mood in a subset of emerging adults, specifically undergraduates attending a private 4-year university. Participants (N = 312) were randomly assigned to one of the following 20-min activities: browse the Internet, passively browse others’ Facebook profiles, actively communicate with others on Facebook via messages/posts, or update their own personal profile on Facebook. Participants also completed questionnaires assessing mood, feelings of envy, and perceived meaningfulness of their time online. The results demonstrated that using Facebook led to significantly worsened mood compared with browsing the Internet, especially when participants passively browsed Facebook. Furthermore, perceptions of meaningfulness, but not feelings of envy, mediated the relationship between online activity and mood. Overall, these findings add to the mounting evidence that social media use may, at times, adversely affect psychological well-being. (PsycINFO Database Record (c) 2019 APA, all rights reserved)