Adolescents are frequent users of social media, with research suggesting both potential harms and positive impacts from use.Black and Hispanic/Latinx youth in particular are both early adopters and high users of social media platforms.However, adolescents-and youth of color in particular-have relatively little say in the design of such platforms.We propose youth participatory action research (YPAR) as a model for informing co-design sessions with representatives of a social networking platform to develop communitybuilding solutions and improve youth developmental outcomes.In a four-months-long study with Black and Hispanic/Latinx teens aged 14-17 (𝑛 = 14), we examined how their sense of engagement and efficacy were altered by actively leading, participating in and contributing to design exercises facilitated by Instagram, one of the world's largest social media sites.Results of pre-and post-surveys indicated a significant increase in teens' civic engagement as well as leadership efficacy.Our results contribute to the understanding of teenagers' expectations and attitudes toward social media and how participatory methods for achieving equity in design can affect change.Theoretical and practical implications are discussed.