In the digital era, social media has become a transformative force in shaping consumer perceptions of sustainable fashion brands. This study examines how strategic social media marketing tailored to demographic and psychographic factors influences purchasing behaviour. Through a mixed-methods approach that combines PLS-SEM analysis of 96 survey responses with a review of industry practices, we investigate the interplay between social advertising, individual consumer traits, and brand image. The study highlights three main points: social advertising effectively builds brand reliability while generating emotional involvement through sustainable marketing messages, and variables such as age, personal beliefs, and technological familiarity influence consumer reactions across different age groups. In contrast, brand image acts as an intermediary between social advertising influence and the extent of purchasing readiness, ranging from 22% to 55%. The study examines new technologies, particularly used for virtual try-ons, which reduce product returns while promoting sustainability-based customer behaviour. Practical implications suggest that brands must integrate authenticity, influencer partnerships, and data-driven personalisation to bridge the attitude-behaviour gap sustainably. This study contributes to ethical marketing literature by empirically validating a mediation model for consumer behaviour in the context of digital sustainability advocacy.