I think you are onto something: When posts and follows are organized by author, authors are incentivized to use eye-grabbing rhetoric, to provoke adrenaline and make people want more, and to get more: More likes! More followers! When posts are organized by topic, contributors can speak freely, but there is less incentive for authors to seek followers through rhetoric, and more incentive to gain community respect by providing useful perspective. Not that the latter can’t happen on author-organized services, but the incentives seem to distract from it. The author-focused services feel noisier and frenetic or schizophrenic, while the subject-focused services feel more focused and useful.