Emerging adults reported more frequent social media use, more active and passive social media behaviors, and lower vaccine uptake compared to middle-aged adults. While stronger COVID-19 vaccine conspiracy beliefs were linked to lower vaccine uptake across all participants, this relationship was unexpectedly stronger for individuals with less social media use and behaviors, potentially reflecting influences from alternative information sources. Additionally, active social media behaviors were associated with stronger COVID-19 vaccine conspiracy beliefs but not with vaccine uptake.