By every metric Threads is mainstream:

a) Top 10 App Store charts in every country.

b) Heavily promoted through Facebook and Instagram.

c) DAUs are higher than X.

That sounds far more like Twitter than Threads. I get so much spam on Twitter now that I hit rate limits reporting it all.

weird, i get a bunch of music and programming stuff on my Threads feed. it's not very deep, but what's on the surface is quite nice and not a bunch of almost-porn. Twitters become half porn though

There is a lot of that.

But there is far more.

Kara Swisher is on Threads, for instance.