BACKGROUND: Global platforms, such as Meta, YouTube, X (formerly Twitter), TikTok, and Telegram, have faced widespread criticisms for facilitating authoritarian repression of dissident voices, especially in the Global South. In response, human rights defenders have increasingly launched advocacy efforts toward the foreign platforms to defend free speech. Despite the varying forms and effects of such transnational efforts, there lacks research that systematically examines their dynamics. METHODS: This study advances a concept of RESULTS: We find that platform advocacy efforts are more likely to generate significant impact if the advocates focus on issues that resonate with Western democracies, promote campaign publicity via prominent international allies, and are able to engage marginalised dissidents. CONCLUSIONS: The research makes important contributions to both the platform governance and transnational advocacy scholarship by underscoring the unique dynamics of Big Tech governance under authoritarianism in the Global South. Methodologically, by strictly limiting the scope of social media processing to publicly available content with carefully selected accounts and keywords, this study showcases a promising big-data design that minimises privacy risks to vulnerable social media users.