The present study aims to examine who benefits most from social media use in terms of friendship closeness, by examining two opposing hypotheses: the rich-get-richer hypothesis versus the poor-get-richer hypothesis. The rich-get-richer hypothesis proposes that especially socially rich adolescents (i.e., adolescents with high existing levels of friendship support and low levels of loneliness) may benefit from social media use in terms of friendship closeness, because their social media use reinforces and strengthens their already existing positive relationships with friends (Kraut et al., 2002; Valkenburg & Peter, 2007). Conversely, the poor-get-richer hypothesis proposes that especially socially poor adolescents (i.e., adolescents with low levels of friendship support and lonely adolescents) benefit from using social media, because social media use may help these adolescents to compensate for their lack of social relationships (Cheng et al., 2019; Frison & Eggermont, 2020). The overall aim of this study will be to compare the empirical evidence for each of these hypotheses by testing them according to modern theories of human development (Granic, 2005; Keijsers & van Roekel, 2019; Molenaar & Campbell, 2009; Sameroff, 2009). We will investigate the short-term social media-induced changes in friendship closeness among socially rich and socially poor adolescents (Aim 1). In addition, we will investigate how these short-term changes accumulate into longer-term changes in friendship closeness across a period of three months (Aim 2). Finally, we will shed light on person-to-person heterogeneity in the short- and longer-term changes in friendship closeness (Aim 3). To meet the aims of this study, we will use data from a larger project on adolescents’ social media use and psychosocial functioning, which employed a state-of-the-art measurement burst design. We will include data from a pre-ESM survey, a three-week experience sampling (ESM) study, and seven biweekly post-ESM surveys among 383 adolescents. We measured the indicators of social richness and social poorness (i.e., friendship support and loneliness) in the pre-ESM survey. In the ESM study, we measured adolescents’ momentary experiences of friendship closeness and social media use in the previous hour. These ESM assessments will enable us to assess the person-specific short-term social media-induced changes in friendship closeness among socially rich and poor adolescents. Finally, to measure adolescents’ longer-term developmental change in friendship closeness, we administered seven biweekly post-ESM assessments of adolescents’ friendship closeness, which will allow us to assess the person-specific developmental change in friendship closeness among socially rich and socially poor adolescents over a period of three months. We will focus on adolescents’ use of the three most popular social media platforms (i.e., Instagram/Snapchat/WhatsApp) with close friends (Pouwels et al., 2021; van Driel et al., 2019)