Mastodon didn’t work. The architecture couldn’t handle Japanese Twitter migrants overwhelming the network and Gab issues that came later. In current form, it is just an on-prem Twitter clone.
I use Mastodon and am definitely a fan, but the rudimentary stats we have (https://the-federation.info/) don't show the growth.
The number of monthly active users fluctuates between 400k-500k this entire year.