Mastodon has roughly 1% as many active users as Twitter. But it feels like much more than 1%. Why is that?
Because we're not a random 1%. People in some communities are much more likely to find Mastodon appealing. For example, at least 30% of the people I follow on Twitter are on here, based on their Twitter bios.
This "homophily" makes it easier for communities to reach critical mass. The downside is that it makes Mastodon more of an echo chamber (on top of the other reasons that make it so).
And just to express my personal biases :) I would argue that #Fediverse platforms aren't all as limiting as #Twitter, giving a variety of interfaces focused on different content forms, and making it much more inviting to creators.
Me, I like to write, so instances without the tight Twitter character limits are SO MUCH MORE conducive to my bothering to post here.