The rocketship of registered accounts in the last 4d is indeed all bots attacking small instances where the admins are asleep at the wheel: https://lemm.ee/post/177673
The real registered user count is like 200k-300k I think. What you have to do is what the active user graphs on those pages, then extrapolate the registered graphs with a similar curve, ignoring the sharp rise when bots started flooding in this week.