Platform organizations bring renewed attention to power disparities and risks in the rise of surveillance capitalism. However, such critical accounts provide a partial understanding of the complexity of surveillance phenomena in such shifting socio-technical and digital environments. Findings from a netnographic investigation of a healthcare platform organization, PatientsLikeMe, unravel how platforms become the locus where multi-level flows of surveillance converge, thereby constituting what we identify as a surveillant assemblage. We develop a comprehensive approach for understanding how platforms constitute a dynamic crossroads of micro, meso and macro surveillance phenomena within and beyond the online communities they create. Our study highlights this surveillant assemblage’s emerging practices and potentially empowering outcomes that enable multi-stakeholder involvement in big data and knowledge generation in healthcare. Broader implications of multi-level surveillance in and through platforms are discussed.