In this study, we drew on an integrated model of objectification and social comparison theories to test the associations between young women’s use of social networking sites and their body image concerns. A sample of 338 undergraduate women, aged 17–25 years, completed online questionnaire measures of engagement in three social networking site activities (browsing or following celebrity, fashion, and beauty sites, browsing or following fitspiration-related content, and placing importance on online “likes” and comments). Also assessed were upward appearance comparison, body surveillance, social appearance anxiety, and two indices of body image concerns (drive for thinness and body dissatisfaction). Structural equation modeling was used to test two competing models, both of which posited social appearance anxiety as the immediate precursor to body image concerns. In line with the integrated objectification-social comparison model, results supported a serial mediation model that comprised significant paths from two of the social networking site activities (browsing or following celebrity, fashion, and beauty sites, and placing importance on online “likes” and comments) through, in turn, upward appearance comparison, body surveillance, and social appearance anxiety, to drive for thinness and body dissatisfaction. Viewing fitspiration-related content was associated with body image concerns directly, rather than indirectly. Findings highlight objectification and appearance comparison factors as targets for future interventions regarding appearance-related social networking site use.