Growing evidence indicates that the diffusion of information and communication technologies (ICTs) can substantially alter the contours of collective violence in developing nations. However, empirical investigations of such effects have generally been hampered by an inability to systematically measure geographic variation in ICT penetration, across multiple technologies and multiple countries. In this article, I show that geo-referenced household surveys can be used to estimate subnational differences in the spatial reach of radio and cellular communications infrastructures in 24 African states. By combining these estimates with geo-referenced measures of the location of disaggregated events of collective violence, I show that there are important differences between centralized ‘mass’ communication technologies – such as radios – that foster vertical linkages between state and society, and decentralized ‘social’ communication technologies – such as cell phones – that foster horizontal linkages between the members of a society. The evidence demonstrates that the geographic reach of mass media penetration generates substantial pacifying effects, while the reach of social media penetration generates substantial increases in collective violence, especially in areas lacking access to mass media infrastructure. I argue that these findings are consistent with a theory of ICT effects which focuses on the strengthening and weakening of economies of scale in the marketplace of ideas.