The number of active Mastodon users — those who've logged in in the last 30 days — has fallen to 1.8 million from a high of 2.6 million. This reflects the fact that while there's a wave of new users after each Musk tantrum, the majority aren't sticking around. Even the 1.8M figure is inflated by new accounts, so I expect a further drop.

Still, the number of active users is over 4 times what it used to be before the Twitter takeover.

Source: https://api.joinmastodon.org/statistics (and Wayback machine).