In terms of users, it'll be Mastodon, the Twitter-alike.

goes looking for numbers

This doesn't indicate whether "active users" is monthly active users (MAU), which is probably the most-common metric for social media, but based on this:

https://mastodon-analytics.com/

There are presently about 750k "active users" on Mastodon.

EDIT: Oh, fediverse.observer does have statistics, and says that theirs are MAU.

According to that, they're at 912k MAU, so maybe that active user count above is something like weekly active.

https://mastodon.fediverse.observer/stats

Here on the Threadiverse:

Lemmy has about 50k MAU.

https://lemmy.fediverse.observer/stats

Piefed about 6k MAU:

https://piefed.fediverse.observer/stats

And Mbin about 900 MAU:

https://mbin.fediverse.observer/stats

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.

I liked piefed until I learnt about the hidden filters. But still, I see their point. There are dozens of lefties here! Dozens!!

Edit: thanks rimu for your reply. I was unaware of that post. Piefed is my fav again :) Somehow I was under the impression that piracy comms were banned.

Not counting Threads, it's probably Misskey.