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: