As measured by https://instances.social/list/old , Mastodon just reached (pinkie to mouth gesture) one million users.
And 52% of them are on the three big Japanese instances (pawoo.net, mstdn.jp, friends.nico). That percentage is down from a peak near 54% earlier, but if you throw in all the other Japanese instances (which are most instances, overall), that side of the network remains a large majority of Mastodon and likely a bare majority of the entire "fediverse." https://mstdn.io/media/ROqRZTsUmbaVq5QHXPk