Preadolescent social media use is normative and could influence mental health. This study investigated: (a) Differences between preadolescent users and non-users of various social media platforms on mental health, (b) unique links between time spent on those platforms, appearance-based activities on social media, and mental health, and (c) the moderating role of biological sex on those relationships.Preadolescent youth (N = 528; 50.9% male) completed online surveys.Users of YouTube, Instagram, and Snapchat reported more body image concerns and eating pathology than non-users, but did not differ on depressive symptoms or social anxiety. Appearance investment uniquely predicted depressive symptoms. Appearance comparisons uniquely predicted all aspects of mental health, with some associations stronger for females than males.Preadolescents could be encouraged to reduce their opportunities to make appearance comparisons and to invest less in their appearance on social media. Preadolescents may benefit from social media intervention programs.