Background Digital mental health interventions present a unique opportunity to address the lack of social connection and loneliness experienced by young people with first-episode psychosis (FEP). The first generation of digital interventions, however, is associated with high attrition rates. Social media presents an opportunity to target this issue. A new generation of digital intervention has harnessed the popularity of social media to both promote engagement and foster social connectedness in youth mental health interventions. Despite their potential, little is known about how young people engage with, and experience, social media–based interventions as well as the optimal design, implementation, and management needed to ensure young people with psychosis receive benefit. Objective This study aimed to explore how young people engage with, and experience, a long-term social media–based mental health intervention designed to address social functioning in individuals with FEP. Methods This qualitative study was based on 12 interviews with young people who used Horyzons, a long-term social media–based mental health intervention, as part of a previous randomized controlled trial. A semistructured phenomenological interview guide with open-ended questions was used to explore young people’s subjective experience of the intervention. All interviews were recorded and transcribed verbatim. Data were analyzed using interpretative phenomenological analysis. Results A total of 4 superordinate themes emerged during the analysis including (1) shared experience as the catalyst for a cocreated social space, (2) the power of peer support, (3) an upbeat environment, and (4) experiences that interrupt being in Horyzons. Conclusions We found that Horyzon’s therapeutic social network fostered a connection and an understanding among young people. It also aided in the creation of an embodied experience that afforded young people with FEP a sense of self-recognition and belonging over the long term. However, although we found that most young people had strong positive experiences of a social connection on Horyzons, we also found that they experienced significant barriers that could substantively interrupt their ability to use the platform. We found that social anxiety, paranoia, internalized stigma, lack of autonomy, and social protocol confusion interfered with young people’s usage of the platform. From a design perspective, digital interventions are flexible and thus equipped to begin addressing these implications by providing customizable and personalized treatment options that account for varying levels of social connection and psychological need that could otherwise interrupt young people’s usage of social media–based interventions.