This thesis examines how Boohoo, Shein, and Zara communicate their sustainability efforts on Instagram and how users react. Despite increased global awareness of sustainability, the fast fashion industry's rapid trend turnover, driven by social media and influencers, fuels consumption and environmental harm. Guided by theories of strategic communication, organizational legitimation, CSR communication, and transparency, the research used critical visual methods, including semiotic and qualitative text analysis. The analysis found that these companies use visual content to appear transparent, shift environmental responsibility onto individuals and the industry, make vague sustainability claims, and partner with influencers for legitimacy. User comments reveal concerns about worker treatment, skepticism towards sustainability efforts as greenwashing, and criticism of influencer partnerships. However, some users legitimize these practices, believing all businesses are equally culpable. The study concludes that while fashion companies recognize the risks they pose and attempt to address them, users are unimpressed by superficial efforts. The gap between companies' staged transparency and users' demand for real change highlights the need for genuine commitment to sustainability. Companies must be transparent and accountable to avoid greenwashing accusations and maintain consumer trust. This growing consumer awareness and empowerment suggest that critical examination by users and authentic sustainability efforts by companies are essential for future legitimacy.