I just graphed Mastodon's MONTHLY ACTIVE USERS each day for the month of November.
At the end of October there were 496K active users, and at the end of November there are 2.6M -- that's 424% growth.
This is just ACTIVE users; there are millions more non-active accounts.
(For comparison, Twitter allegedly has 396.5M monthly active users, or about 152x the active userbase of Mastodon.)