If the users chooses what they are shown and the order they are shown in, then it's fine. If the platform chooses, then it's not, because they will always choose what creates the most engagement.

Is heroin the same as beer? Who even knows.

That's the problem, recommendation algorithms on networks beyond a certain scale should be publicly auditable.

If they're not, I should be able to opt-out of them.

I think I would be fine with a positive enumeration. Some ideas for serving content:

    - purely random
    - sponsored but without user tracking (like old school TV ads)
       - sponsored for user selected geographical area feed
       - sponsored for user current location geographical area feed
    - follow "friends" or influencers
    - purely timeline

    - discussion boards
      - timeline (IRC like)
      - threaded
      - user votes (not magic platform votes)

    - follow keywords