Purpose To determine how social media platform and cancer content is associated with the presence of social support in responses to young adult cancer caregivers' (YACC) posts.Design We retrospectively collected YACC's Facebook and/or Instagram posts and all responses from the first six months of caregiving.Sample Eligible YACC were 18-39, caring for a cancer patient diagnosed 6 months-5 years prior, spoke English, and used social media weekly.Methods Social media posts and responses were manually coded for five social support types, then transformed to depict the proportion of responses per post representing each type of support. Using mixed-effects models, we compared the distributions of responses with social support types by platform (Facebook vs. Instagram) and cancer content (no vs. yes).Findings More responses contained emotional support on Instagram than Facebook (B = 0.25, Standard Error (SE)=0.09, p = 0.007). More responses with cancer content contained validation support (B = 0.20, SE = 0.07, p = 0.002), but fewer contained emotional (B=-0.17, SE = 0.07, p = 0.02) and instrumental support (B=-0.06, SE = 0.02, p = 0.001) than posts without cancer content.Conclusions Studying the responsiveness of social media followers by platform and cancer content provides a foundation for intervention development.Implications for psychosocial providers Emphasizing the suitability of different social media platforms for particular support seeking behaviors is essential.