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.

Mastodon. Hands down.

Mastodon has more monthly active users than the entire rest of the Fediverse combined. There are lots of people for whom the Fediverse equals Mastodon. Most of the time, when someone hears about the Fediverse for the first time, it's explained to them as only Mastodon, allegedly because that's easier to understand. And there are vastly more people who have heard of Mastodon, but not the Fediverse, than who have heard of any other Fediverse software, but not the Fediverse.

There are tons of phone apps made for Mastodon and only Mastodon. There are websites that have "Fedi" in their name, but that are built hard against only Mastodon, and if they're compatible with anything else, it's by chance. diaspora* (remember 2010 when those four kids announced a free, open-source, decentralised "Facebook killer", asked for $12,000 of crowdfunding and got over $200,000) refuses to join the Fediverse because, according to one dev, Fediverse developers don't "implement ActivityPub", they "implement Mastodon".

Lots of people say that claiming that Mastodon is the Fediverse is justified.

Here are the numbers from the FediIndex:

• Most active instances: Mastodon (36.98%; Lemmy is 9th with 1.11%)
• Most accounts: Mastodon (51.16%; Lemmy is 7th with 1.77%)
• Most monthly active accounts (not counting Misskey and GoToSocial whose stats are off): Mastodon (51.17%; Lemmy is 5th with 2.37%)
• Most posts: Mastodon (60.48%; Lemmy is 10th with 0.54%)