I find the plateau quite puzzling (lemmy.world, but the total looks very similar):

Image

There was quite a steep increase, and then it suddenly stopped.

I would rather expect it to slow down, than to stop that abruptly.

We’re looking at a fairly large group of people making a decision to create an account on Lemmy. There are plenty of reasons to expect it to be fuzzy. Even if they all responded to one particular event in time, some would have done so immediately, others the next day, few more even later.