@NickBohle @tchambers @pfefferle None of them are super accurate, I'm afraid. There's overlap between the Active Installation number and the number of sites hosted by a8c. Anything on FediDB, and sites like it, relies on pinging instances, so there's a huge long tail of low-traffic sites that they don't pick up.
Monthly Active Users is probably a more meaningful metric—we're in this to make it a rewarding experience that folks want to come back to!