Bluesky has now 15 million total users (how many active?)
Mastodon monthly active usage has dropped below 1 million.
YMMV depending on your region but my Play Store "social" chart is currently showing Bluesky at #1, then TikTok, Threads, Instagram, Facebook, X, and Reddit at #7.
It is if they have a Threads account and interact with any Threads content on the Instagram app, which is extremely easy to do even accidentally, because they shove it into the Instagram feed and make it look like Instagram content.
I would not be surprised if a large chunk - maybe even the majority - of "Threads users" interact with it exclusively through the Instagram app, with many of them not even fully aware that it's nominally a separate product.
And also into your notifications now.
Of course Instagram helps, because they keep showing Threads posts on Instagram, notifications, etc, so people download and create an account, but is not automatic.