[buildfocus]

Creator of Mastodon has 330k. Not sure if he's #1 but he's a strong contender: https://mastodon.social/@Gargron

EDIT: https://docs.google.com/spreadsheets/u/0/d/1cpUKkoT1MUn8_xM4... has a list of the big ones, looks like https://mastodon.social/mastodon is top at 800k.

> I figured there was less than 100K people even on Mastodon

https://mastodon.fediverse.observer/stats says currently 2 million active monthly users, 9 million registered. That's just for Mastodon, so add another couple of million for the rest of the Fediverse.