The structure that is designed to foster the userbase. Social Media is designed to bring in as many people as possible, in a very open-ended platform. The older types of platforms, like these forums, are designed around a community. Social Media wants as many people as possible; forums want specific people. Forums and the like have an upper limit before they break, and are best when there's a limited, distinct group of people; Social Media has no ceiling, and fails if it lacks critical mass.
It is the societal difference between having agency and being part of a community, and being oversocialized and atomized.