It is probably due to a number of people stopping using their alts after some instance hopping.
Also a few people who came to see how it was, and weren't attracted enough to become regular visitors.
Curious to see at which number we'll stabilize.
Next peak will probably happen after either major features release (e.g. exhaustive mod tools allowing reluctant communities to move from Reddit) or the next Reddit fuck up (e.g. removing old.reddit)
Stats on each server: https://lemmy.fediverse.observer/list