Where would you find these metrics? Looking at the federated timeline, I see posts appearing at a pretty fast rate (more than one per second), though most of them are in Japanese.
go and ask everyone you meet in person today if they've heard of mastodon
I bet that few of them will reply that it is an extinct animal, some others that it is a cool band, and most of them that they have never heard of such a thing.
It's very popular in Japan because of the "lolicon" phenomenon (cultural sexy drawings of youth people), which is not accepted by Twitter because of the occidental culture in which this is a taboo.