X has declined yes, but threads is growing. Loads more joining recently as well. Most seem to move from FB to threads because they have a Meta account, so have an Insta account soooooo.... Threads is a dumbed down X (I can feel the heat I'm gonna get for that) Meta is by far and away the largest for users - we all know it and it means that promoting threads inside Insta and FB means people will see what it is like. Here in the UK it is refreshingly free of ads, is quick to post/reply/interact and feels new. How long that will last is anybodys guess as per usual

Instagram is extremely popular, and it's heavily promoted inside of there, with Threads content embedded to almost look like Instagram content but when you tap on it it hops you over to Threads. I'm not surprised that they've been able to build a user base while X declines