<p>Social media has emerged as an important part of daily life for many people, with potential implications for subjective well-being. In this cross-sectional study, we investigated whether time spent on social media, social media stress, and social media self-regulation failure are associated with emerging adults’ subjective well-being (positive affect, negative affect, and life satisfaction) and tested whether these links depended on participants’ Big Five personality traits (extraversion, agreeableness, conscientiousness, emotional stability, and autonomy). A representative community sample of emerging adults (N = 343; 42.3% men; M_age = 24.82 years) completed self-report questionnaires. Consistent with the “Goldilocks hypothesis”, both too much and too little time spent on social media were associated with greater negative affect, with 45 minutes per day predicting the lowest negative affect. Moderation analyses revealed that time spent on social media was related to lower positive affect for participants low on extraversion. Social media self-regulation failure was related to lower positive and higher negative affect for participants with low agreeableness. Social media stress related to less negative affect for participants with low emotional stability, and to more negative affect for people with high emotional stability. These findings suggest that the impact of social media on emerging adults’ well-being may depend in part on their personality traits.</p>