So this is charted over months, so presumably 'active users' are those that log-in sometimes.
So what's 'number of users'? How can it go down near the end? Did they purge accounts? Otherwise, shouldn't it only go up?
I assume this just aggregates publicly reported data from instances. So in addition to accounts being deleted (which happens all the time of course), some instances could have stopped exposing that information publicly, some instances fake this data, some instances went away (if a large-ish instance closes you'd expect new accounts of its user pop up elsewhere over some time, and then a big chunk disappearing), maybe were removed from the dataset because their data is fake, ...