Eighteen earlier studies have investigated the associations between social media use (SMU) and adolescents' self-esteem, finding weak effects and inconsistent results. A viable hypothesis for these mixed findings is that the effect of SMU differs from adolescent to adolescent. To test this hypothesis, we conducted a preregistered three-week experience sampling study among 387 adolescents (13-15 years, 54% girls). Each adolescent reported on his/her SMU and self-esteem six times per day (126 assessments per participant; 34,930 in total). Using a person-specific, N = 1 method of analysis (Dynamic Structural Equation Modeling), we found that the majority of adolescents (88%) experienced no or very small effects of SMU on self-esteem (-.10 < β <. 10), whereas 4% experienced positive (.10 ≤ β ≤. 17) and 8% negative effects (-.21 ≤ β ≤ -.10). Our results suggest that person-specific effects can no longer be ignored in future media effects theories and research.