Three 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.)
@going_to_maine I'm still here...but mostly posting on other instances. So point no. 2 could be a big part of it.
No. 3 is probably also pretty common. Everyone gets excited about The Next Big Thing™. Then, after a bit, many people drop off and go back to whatever they were doing before.