this site started recently with really good granular data. for a user to count as active it needs to have made an activity (like/follow/post) in that timeperiod.
accurate monthly active user data should become available in like 3 weeks.
they are currently just below 500k daily active users
https://bskycharts.edavis.dev/edavis.dev/bskycharts.edavis.dev/bsky_users_total.html