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

[–] Grumpy404@lemmy.zip 6 points 3 days ago (1 children)

Good to know, ive never used twitter because the horrible things ive heard about it, but how does this platform compare? im trying to learn towards using more free and open source software, but i would say im failing at that atleast the open source ones, i still use a ton of free software.

[–] pmk@piefed.ca 9 points 2 days ago

Mastodon is more centered around specific people you choose to follow, everything is in chronological order, regardless of topic or if it's a reply to someone else. Lemmy has threaded replies and the topic is more in focus.