i don’t have any data on other fediverse platforms at hand rn, but from memory IIRC mastodon has close to 1 million users, and it’s by far the biggest chunk of the fediverse.

• mastodon is people-centric, where you have person/user as the central element of organization, and they can have posts.
• lemmy is content-centric, where you have communities as the central element of organization, and they can have posts.

AFAIK, content-centric networks are only lemmy and piefed as of today, modeled after Reddit. lemmy is bigger (i think) with 30K (active) users.