If you want to watch the numbers yourself, FediDB (fedidb.org) and fediverse.observer list each software by number of servers and monthly active users. MAU is a fairer measure than total accounts, since lots of accounts are dead.

Roughly: Mastodon is by far the biggest, for microblogging. On the Reddit-style side you're on now (people call it the "threadiverse"), Lemmy is the largest, with PieFed and Mbin as the main alternatives, and they all federate with each other, so you can follow the same communities from any of them. Misskey and its forks are also big, especially with Japanese users.

Since you're learning: make an account on one and follow a person or community that lives on a different software. Seeing a Mastodon post show up here as a thread is the moment federation clicks.