I'm pretty sure it should be "in a tweet" (perhaps because a tweet is short?). And perhaps also the same for a Facebook post?
But for a long Reddit/Stack Exchange post (perhaps with many replies and answers), should/could we use in/on/at?
Examples:
As was mentioned ___ this Stack Exchange post, ...
As stated in the selected answer ___ this Stack Exchange post, ...
See the discussion ___ this Stack Exchange answer.
See the discussion ___ this Stack Exchange post.
See the many discussions ___ English Stack Exchange.
See the many discussions ___ r/English.
(What are the deeper explanations for why we might choose one preposition over another here?)