While social media has been praised for youth engagement with science, evidence of its impacts remains fragmented. This scoping review reports on the impacts of social-media-based science communication on young audiences. A PRISMA-guided database search yielded 2,257 articles, which were screened to include only empirical articles studying social media’s behavioral, attitudinal, and cognitive impacts on audiences, including youth, in science or health contexts. Using Directed Qualitative Content Analysis, the impacts desired, measured, and observed were categorized in the 35 remaining articles. The most desired and measured impact was knowledge gain, while the most observed outcomes were interest and trust in science. Many studies desired specific impacts but failed to measure them. Impactful content was relevant, visually appealing, and emotionally engaging. However, studies recognized that unreliable actors may also manipulate these characteristics to spread misinformation. While many science communicators assume the importance of social-media-based science communication for young audiences, evidence of observed outcomes is limited and specific to platforms and topics.