From what I remember, Misskey fills the niche that Mastodon does and it’s been popular in Japan for a lot longer than mastodon was popular elsewhere
Also I’d be curious what the breakdown of Japanese users is across the three. Overall user counts are one thing, but maybe the number of Japanese users matches that order (Misskey > Bluesky > Mastodon)
When something is posted on Mastodon, Pixelfed, etc., I can access it without an account.
When something is on Twitter/X, Instagram, etc., I can’t access it (either easily, or at all) unless I make an account and sign in with it.
Important information should not be on a platform where it is locked away like this.