Online opinion formation has received much scholarly attention since the mass proliferation of social networks. Inter alia, online opinions have been viewed as a new part of public deliberation. However, the pre‐Internet era's vision on deliberation imposes extremely high demands on users as deliberators. We argue that opinion formation online neither pursues the goals nor follows the rules of institutionalized consensus‐oriented round‐table deliberative processes. Moreover, the growing academic evidence shows that opinion formation online is predominantly cumulative, not deliberative in nature. Thus, we introduce the concept of cumulative deliberation as an alternative and addition to classic institutional deliberation and argue that it describes opinion formation online more precisely. Importantly, it allows for two crucial additions to the deliberation theory, which are the use of systemic approaches to measuring and predicting public opinion and new normativity that sees a user as an initially neutral discussion unit. It also allows for healthier distinction between “natural” user communication and intentional counter‐deliberative distortions in online communication, like computational propaganda or cyberbullying. We end up with suggesting a research agenda on cumulative deliberation.