I'm not sure Threads ever had that much of a spike in usage beyond the first day of new accounts. Got the impression a huge number of people curious about it signed up due to the very easy onboarding if you already had an instagram, looked around a little and then never went back.

Outside of the tiny number of Threads users who didn't have an Instagram account beforehand, the act of registering to BlueSky is a far more engaged move than Threads ever had imo.