I would disagree. It is community building in a different construct. There is no /r/X or group structure built in. I am only Twitter because I design board games. I only interact with board game community accounts and posts. The community is diverse, varied and thriving. If I was trying to get 100K followers to argue about politics maybe I'd have a bad time. I'm just trying to say that there is a lot of community building but it is self-directed not controlled by the platform.