The digital era has ushered in a new collective action landscape that scholars are now trying to understand. To date, scholars have assumed homogeneity in the effect of ICT (information and communications technology) on collective action. In this article, I show that the effect of ICT on collective action is heterogeneous, and that it is sensitive to national and cultural context by conducting a cross-case comparison of Japan and South Korea. These two countries present a particularly illuminating pair of cases, with Korea showing consistently high levels of collective action and Japan showing consistently low levels of collective action despite the introduction and diffusion of ICT. I argue that this participation gap is in part due to differences in the way social network dynamics operate in the two countries. I show that social network size is more strongly associated with participation in protests, boycotts, and petitions (PBP) in Korea. Additionally, I find that social network heterogeneity has a positive association with participation in political groups in Korea, whereas the two variables have a negative association in Japan. I conclude by discussing the broader implications for our understanding of the relationship between collective action, social networks, and culture.