This paper seeks to argue the relevance of knowledge management (KM) to the development of social capital, and to enhancing the capacity to take effective action in human social systems. The study applies a pluralistic definition of knowledge (including subjective beliefs in minds and objective claims expressed in language) to show that most forms of social capital reduce to knowledge. First, social capital mostly comprises knowledge (trust, beliefs, rules, and norms). Second, the capacity to individually and collectively learn (in networks) is therefore arguably the most important form of social capital, even if rarely acknowledged as such in the literature. Third, because of the importance of learning and innovation to the production of social capital in society and organizations, KM has an important role to play in related development efforts. The paper introduces social capital constructivism. Practicing it to strengthen social capital can enhance the capacity to take effective action in human social systems. This points to a new value proposition and functional orientation for KM: to enhance the human capacity to take effective action by fostering the growth and development of social capital. The thesis reveals social capital as consisting mostly of knowledge, and shows how KM can enhance a human capacity to take effective action in social systems by fostering the production of social capital itself.