I can't judge the rate because I never precisely knew how much people actually tweeted, only how much Twitter wanted to show me.

As for not deleting accounts, did people actually delete their MySpace accounts? Or just slowly start using them less over time?

(FWIW I have just unfollowed everyone I was following and will keep an account purely to reduce login nagging when following a Twitter link.)

I think what's going on in those cases is that people are heavily invested in Twitter, believing their bulk audience is there, but (understandably) want to hedge their bets. Lots of them are posting the same stuff to both Twitter and Mastodon (tooling exists) and they'll wait and see how it pans out. As they see more interaction on Mastodon, perhaps they'll migrate their focus, too.

Interesting time, for sure.

I think what you're seeing is bridges like https://moa.party/ which just tweet what people post on Mastodon. Same content but not the same engagement.