Background: Globally, suboptimal dietary choices are a leading cause of noncommunicable diseases. Evidence for effective interventions to address these behaviors, particularly in young adults, is limited. Given the substantial time young adults spend in using social media, there is interest in understanding the current and potential role of these platforms in shaping dietary behavior. Objective: This study aims to explore the influence of social media on young adults' dietary behaviors. Methods: We recruited 234 young adults aged 18-24 years and living in Australia, using market and social research panels. We applied a digital ethnography approach to collect data from web-based conversations in a series of forums, where participants responded to different health-themed questions related to health behavior change and persuasion on social media. We conducted a qualitative thematic analysis. Results: Participants described how social media influenced their decisions to change their health behaviors. Access to social support and health information through web-based communities was juxtaposed with exposure to highly persuasive fast-food advertisements. Some participants expressed that exposure to web-based health-focused content induced feelings of guilt about their behavior, which was more prominent among women. Fast-food advertisements were discussed as a contributor to poor health behaviors and indicated as a major barrier to change. Conclusions: Young adults reported that social media is highly persuasive toward dietary behavior through different pathways of social influence. This suggests that social norms on the web are an important aspect of changing young adults' health behaviors. The commercialization of social media also encourages poor health behaviors, largely through fast-food advertisements. Future social media-delivered dietary interventions should acknowledge the social and environmental factors that challenge the ability of young adults to make individual health behavior improvements. Care should also be taken to ensure that future interventions do not further elicit guilt in a way that contributes to poor mental health within this community.