Adolescents actively use social media, which engages them cognitively, emotionally, and behaviorally. However, the underlying psychological mechanisms of engagement have not been adequately addressed. The present study examined adolescents’ psychological processes as these develop in their everyday interactions via social media. The sample comprised six focus groups with 42 adolescents from UK-based schools. Data were analyzed using constructivist grounded theory. The resulting concepts related to individual, social, and structurally related processes, highlighting a synergy between the processes underlying use and a gradual reduction of control as individual, social, and structurally led processes emerge, conceptualized as the ‘control model’ of social media engagement. The findings highlight a controlling aspect in engagement and a dynamic interplay between the processes as mutually determining the quality and the intensity of the interaction. Recommendations are provided for examining control as a main emotional, cognitive, and behavioral mechanism in problematic and/or addictive social media and smartphone use.