At least Twitter mostly still goes by post date.
Facebook made that a separate feed that you need to practically dig for to find now; it's not actually an option for the news feed itself anymore.
Everything there is filtered through their algorithm that decides what you're mostly likely to enjoy, and posts will show up in the feed again at a later time if there's activity on them from friends.
All that plus reloading the entire feed when exiting the Post comments (on the mobile app specifically), making it hard to find the same post again if you're trying to share it.
I really hope Twitter doesn't go any farther than it has away from a linear feed. There is at least some sense left in how they built it.
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