If you dislike a post, you most likely have a reason for it, maybe you disagree or have another specific reason in which case it would be better for you to write up a post explaining why you dislike it anyway so I doubt the dislike function would be a good feature to have.
The Like/thank button is different because in a way it prevents people of simply reposting the same crap and using it in a way of showing your appreciation towards a post or your agreement, something that doesn't always require a follow-up post unless you feel like adding something that you absolutely feel is missing.
Plus I can already imagine some people just dislike posts left & right because they dislike a user whereas right now they would atleast need to have a valid reason to prove that they disagree with a certain post so it isn't as easy

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