Informed by the interaction of person-affect-cognition-execution (I-PACE) theory, the present studies examined the association between peer rejection, peer popularity, and social media addiction (SMA) at both between-person and within-person levels. Two distinct processes, the fear-driven/compensation-seeking process and the reward-driven process were explored. In Study 1, using a cross-sectional sample of high school students (N = 318), both processes were supported via different cognitive mediators. Support for the fear-driven/compensation-seeking process was demonstrated by finding that avoidance expectancy was a significant cognitive mediator between peer-nominated rejection and SMA. In turn, the reward-driven process was supported by the significant mediation of reward expectancy between peer-nominated popularity and SMA. In Study 2, using ecological momentary assessment with college students (N = 54), we found the fear-driven/compensation-seeking process partially supported through both between-person and within-person mediations. Specifically, negative affect and social media craving were two affective mediators that linked peer rejection and addictive social media use behaviors. On the other hand, the reward-driven process was predominantly supported by within-person mediations, in which positive affect and social media craving were found to be mediators of the relationship between peer popularity and addictive social media use behaviors. The results underscore that adolescents experiencing rejection tend to use social media to avoid negative feelings and compensate for interpersonal deficits, while adolescents experiencing popularity tend to use social media to maintain positive feelings and gain social rewards. Implications for the assessment, case formulation, and treatment of SMA in counseling practice are discussed. (PsycInfo Database Record (c) 2024 APA, all rights reserved).