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
I think the honest answer is to become active and solicit on Mastodon. Those users are not only far more open to the pitch of “Mastodon but with threaded discussions” but are far more legitimately engaged and active than Reddit users.
EDIT: Not to mention they can literally participate from their existing accounts. Super easy to get your foot in the door.