I checked and it looks like the graph is right but the annotations are wrong. The leveling out part is when Talio shut down sign up:
Then the hack happened:
Then FNG started:
So the steepest part of the incline is probably some combination of Forsaken Angel or Uin13 posting a link to nerfhaven on their youtube channel, a different admin taking over membership approval and possibly approving more accounts, and possibly also automated spambots attempting to join and never getting purged. Then it levels off when Talio stopped accepting new members without an email application. Then when I opened membership again with the FNG system, it steadily increased at about the same rate as it did in the mid-2000s. It started to level out around mid 2017. Membership numbers during that time are probably more to do with things like the Captcha on the sign up page breaking. You'd have to look at new posts to get a real idea of activity on the site.
A couple of notes about the way you're collecting info:
Registration date is not the date members were approved under the old system or graduated to members under the fng system. It's just the date they signed up.
Your numbers only include people who were members when you collected the data. Members in the 'waiting' group and FNGs who never posted got purged if they hadn't logged in within a year. Anyone left after that by definition must have posted at some point, or at least actively logged in recently. I performed another one of these purges within the last couple of weeks.
Anyway, I've interpreted data that you painstakingly assembled about the history of a dead internet forum, and I read my own writing from 10 years ago. Now I need a beer and a shower, so thanks for that.