Going T. MaineThree possibilities : a lot of mefites have blocked me (cool if you did! It's a free federation), outdoor have migrated to be accounts, or Mastodon has experienced some user decay. (could also be a tasty blend of all three.)
Josh "cortex" Millard@going_to_maine points two and three both feel really intuitive for masto's engagement curve. It's notable that point two is even a thing, vs. e.g. The Great Elloing in which everybody disappeared not to other instances but back to Not Ello.