They’re doing some dodgy stuff with your data, that’s for sure. But the sign-up is smooth. Instagram has a LOT of users, and Meta is leveraging that and making it super easy to share to IG that you have made said threads account (and if you don’t, they will do it for you). The FOMO is real.
They have taken the biggest challenge to use a new social media platform, rebuilding your spidery web of follows, mutuals, weird interests, and friends, and made it basically transparent. It’s a UX marvel. The sign-up numbers reflect that IMO.