IIRC twitter was originally called a "micro-blogging" platform, and "re-tweeting" and replying to tweets came later. At that point it became social media.
Something like RSS doesn't work for direct messages, but it does still allow for you and I to post to our feeds. Nothing stops it from going a step further and acting much like twitter, we all post to our own site but they can be short messages and they can reference a post on someone else's site as "replying to" or similar.
1. Users are not given a choice (well, accept to leave, and I know many people who have).
2. This is measured on time spent, not user satisfaction.
In practice, we demonstrated that algorithms designed to be addictive are addictive (success?), not that there’s an actual preference.