This article discusses the role of giant social media corporations Facebook, Google (YouTube), and Twitter in counter-terrorism and countering violent extremisms (CT/CVEs). Based on a qualitative investigation mobilizing corporate communications as well as a collection of interviews with European stakeholders, it argues that these firms have become actors in this policy area of what is traditionally considered high politics, through their fundamental role in establishing and enforcing the nascent global governance regime on terrorist communications. Since the emergence of the self-proclaimed Islamic State of Iraq and Syria (ISIS), the studied firms have displayed agency and creativity in their appropriation of this new responsibility, effectively going beyond what was legally required of them. After contextualizing and questioning their involvement, motivated by terrorist exploitation of their services, reputational pressure and the threat of legislation, the article provides an analysis of the firms’ self-regulated commitment to CT/CVE through policymaking, content moderation, human resources, and private multilateralism.