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The exact same approach can be taken on Twitter, though.
Yes, Twitter does provide the additional official option, but this out-of-band approach is also available, just as a matter of knowledge and using available technology.
It doesn't rely on any particular ActivityPub functionality.
@volkris @briankrebs @jerry I see. Thanks for your comment. But what is the paid option intended for? I don't see any use in offering the same feature as paid service and for free.