I keep seeing this analogy and unfortunately that's not how email servers work so it never really helps honestly. The servers are the To: fields, not the From: fields. And there's also no real analogy about privacy. With most email providers the intent isn't that everyone reads everyone else's email. So frankly I really don't know what insight this is supposed to provide if it doesn't behave like email.
And there's a big safety difference. With something like Bluesky you have to trust the server admins to behave. With ActivityPub you have to trust each and every user of the service. Which is why server admins get shirty about whether they will forward messages to or from other servers. That whole situation doesn't really exist with email. It's not like you have create a Hotmail account because Gmail has decided to defederate with Google or whatever.
It's still up 55% vs one year ago. Yeah it's fallen from its post-election run up which peaked around the new year, but frankly it just looks like it's gone back to pre-election value and trend.