In a large Australian cohort, adolescent well-being showed a U-shaped relationship with after-school social media use, with moderate use associated with the most favorable outcomes. For pediatricians, the key clinical takeaway is developmental nuance: high use was linked to poorer well-being in early to mid-adolescence (especially girls in grades 7–9), while complete nonuse became increasingly associated with low well-being in later adolescence, particularly among boys. These findings support counseling families toward balanced, age-appropriate social media engagement, emphasizing moderation and social connection rather than strict abstinence or time-only limits.