This chapter examines the relationship between social media and violent extremism through a broadly affordances-based analytical framework. After defining social media and affordances, this chapter traces three historical phases of extremist engagement with “mainstream” platforms: an early permissive period characterised by high visibility and participatory experimentation (c.2006–2015); a phase of intensified moderation, deplatforming, and adaptive migration (c.2016–2022); and a more recent period marked by regulatory intervention, platform backsliding, and generative artificial intelligence. Drawing on examples from both jihadist and extreme-right social media content and activity, this chapter then analyses how affordances such as algorithmic amplification, networked community formation, participatory content production, and monetisation have shaped extremist visibility, resilience, and adaptation over time. The conclusion points out that, while social media remains a central node in contemporary extremist ecologies, its dominance in the medium to longer term should not be assumed.