In recent years, platforms like Twitter (now X), Facebook, and WhatsApp have reshaped civic engagement by enabling real-time communication, agenda-setting, and increased citizen participation in governance. It is on this premise that this study investigated the impact of social media on public policy formulation and implementation in Nigeria, focusing on its opportunities and challenges for participatory governance. Adopting the Agenda Setting Theory, originally developed by McCombs and Shaw (1972), the study employed a qualitative research design, relying on secondary sources such as scholarly literature, government reports, and digital media content. Data were analysed thematically to uncover patterns in the use of social media for policy influence. Findings revealed that social media has become a crucial space for civic activism, policy feedback, and digital accountability. It has facilitated movements such as #EndSARS, improved transparency in budget tracking, and enhanced government-citizen communication during crises like the COVID-19 pandemic. However, the study also identified critical challenges including misinformation, online manipulation, limited digital access, and heavy-handed regulatory responses, all of which hinder the full integration of social media into the policy process. The study concluded that while social media holds significant potential to democratize public policymaking in Nigeria, its impact remains constrained by structural, technological, and political barriers. The study made recommendations that: media literacy education should be scaled up nationwide; inclusive digital infrastructure should be prioritized; regulatory frameworks must balance control with digital rights; and public agencies should institutionalize social media as a formal feedback and engagement tool for policy development.