On https://the-federation.info/platform/73 I've noticed a total users dip on 2022-11-04, did some instance closed?
I noticed the same, and my hypothesis is that after a certain time inactive accounts are deleted. The accounts seem to spike a few months before that "crash" in user count. Also, if you look at "active in the last month" you see that many of those new joiners didn't remain active even for a month.