Initial evidence suggests that sharing positive news with others on social media may have short-term emotional benefits for young people. However, little is currently known about whether these capitalization processes have long-term implications for adolescent well-being. Drawing upon the broaden-and-build theory of positive psychology, this study examined the longitudinal reciprocal relationship between capitalization on social media and adolescent life satisfaction. The moderating role of trait optimism was also determined. Three-wave panel data were collected from Belgian adolescents between January and October 2020 (N = 1,125; M age = 15.38, SD = 1.44; Girls = 48.4%), and data were analyzed using a random intercept cross-lagged panel model. No significant cross-lagged relationships were found between capitalization on social media and adolescent life satisfaction at the within-person level. Moreover, the relationships were not moderated by trait optimism. Null results suggest that the antecedents and effects of capitalization on social media may be momentary in nature rather than manifested in long-term associations. Findings therefore add to the limited literature regarding social media’s positive role in adolescent well-being.