"Alice's followers" is the way most social networks work with private X, Facebook, Instagram. It lets Alice ask questions or share private info with people she trusts and cares about, and lets them discuss amongst themselves. It is really the best way to use social networks.
"Bob's followers" is the literalist version, with the worst possible dynamics. "You should reply to a followers-only post with a followers-only post" retains the same UI choice while completely changing the audience. Most of the other people who read Alice's post won't see Bob's comments. Bob's followers who don't follow Alice won't understand the context of his post, and won't be able to read Alice's post. It also violates Alice's privacy to share a response to her question with strangers.
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