Social Media Use and Well-Being Across Adolescent Development.

Importance: Social media's association with adolescent well-being remains debated. Heavy use has been associated with distress, while abstinence may cause missed connections. Objective: To investigate 3-year longitudinal associations between after-school social media use and adolescent well-being using a large longitudinal cohort dataset modeled within a repeated cross-sectional framework. Design, Setting, and Participants: This cohort study included Australian students in grades 4 through 12 (2019-2022). After-school social media use was self-reported and grouped as none, moderate, or highest. Well-being was assessed using 8 validated indicators (eg, happiness, life satisfaction, emotional regulation), dichotomized as high vs low. Well-being was assessed concurrently with social media use during the annual school-based survey in each year of data collection. Data analysis was conducted from June to July 2025. Exposures: Self-reported after-school social media use between 3 pm and 6 pm (weekdays), classified into 3 categories: none (0 h/wk), moderate (>0 to <12.5 h/wk), and highest (≥12.5 h/wk). Main Outcomes and Measures: The primary outcome was overall well-being, measured as the mean score across 8 validated domains (happiness, optimism, life satisfaction, worry, sadness, perseverance, emotional regulation, and cognitive engagement), dichotomized as high vs low (<3 on a scale of 1-5). Secondary outcomes were each individual well-being indicator, similarly dichotomized. Mixed-effects logistic models were used for analyses, stratified by sex and adjusted for demographic covariates. Results: The analytic sample included 100 991 adolescents, contributing 173 533 observations (86 582 [49.9%] observations from female participants; mean [SD] age, 13.5 [2.2] years). A U-shaped association was observed between after-school social media use and well-being. Compared with moderate users, adolescents with the highest use had greater odds of low well-being (grades 7-9, girls: odds ratio [OR], 3.13 [95% CI, 2.88-3.39]; boys: OR, 2.25 [95% CI, 1.86-2.72]), while nonusers also had higher odds of low well-being in later adolescence (grades 10-12, girls: OR, 1.79 [95% CI, 1.41-2.27]; boys: OR, 3.00 [95% CI, 2.01-4.46]). These patterns were consistent across survey years and robust to sensitivity analyses. Conclusions and Relevance: In this cohort study of students in grades 4 through 12, social media's association with adolescent well-being was complex and nonlinear, varying by age and sex. While heavy use was associated with poorer well-being and abstinence sometimes coincided with less favorable outcomes, these findings are observational and should be interpreted cautiously.