<p>At the intersection of rapidly advancing digital technologies and growing social-ecological concerns, the role of social platform–based enterprises is increasingly prominent. These platforms face complex legitimacy challenges. Their survival depends on achieving sustained user growth––a process that can, in turn, create dynamics that pull them away from their social mission. This study investigates how social platform–based enterprises sustain legitimacy as a balance between user growth and social mission. Through an in-depth case study of a food waste platform that connects consumers with food businesses offering unsold food surpluses, we identify four key mechanisms that, in their interaction, enable the platform to sustain legitimacy: transactional work, legitimacy guardrails, selective (de)legitimation, and affective engagement. This research advances understanding of legitimacy as a relational, affective, and multi-level phenomenon involving a plurality of actors and strategies, shedding light on how social enterprises emerge in the era of platformization.</p>