I saw a discussion the other day that may have been part of it. They were pointing out that many users when first joining lemmy created accounts on multiple instances. Whether they needed to find a community they preferred or do to ddos attacks and other issues causing outages a lot of those accounts have likely become inactive more as time went by. So the numbers may not be decreasing so much as been inflated by individuals coming up as multiple active users early on. I haven’t looked into how they are verifying what users are active though, so it could be wrong. If someone knows more please respond.