In July 2018, Bulgaria’s Constitutional Court rejected the Council of Europe Convention on Preventing and Combating Violence Against Women and Domestic Violence (better known as the Istanbul Convention). This rejection came following a wave of civic activism, which scholars characterized as the first anti-gender campaign in Bulgaria. Three years after the Constitutional Court’s decision, the Istanbul Convention continues to occupy online and offline public space, still provoking controversy and sparking heated debates. Employing critical discourse analysis (CDA), this thesis analyzes comments under Facebook posts mentioning the Istanbul Convention between 2017 and 2021. The posts appear on the Facebook pages of two ideologically opposing civil organizations that actively participated in the debate around the Convention’s ratification. Using Reyes’ (2011) concept of de/legitimization discourse, this paper analyses the online discussion around the Convention, exploring how de/legitimization discourses are utilized by the two ideologically opposing communities. To account for the role of Facebook as a discursive practice, the study further explores how the architectures and affordances of Facebook as a platform contribute to the polarization of the IC discussion. The analysis reveals the instrumentality of de/legitimization discourses for constructing pro- and anti-IC activist groups, for redefining the meaning of ‘gender’ in Bulgarian society, and later for transforming the meaning of the phrase ‘the Istanbul Convention’ to outgrow a reference to one document and become a signifier of values. Additionally, the study highlights three types of Facebook affordances that were found to affect the discussion’s polarization: identity, social, and functional.