I saw these numbers recently too and I honestly think they are fraudulent. Are they counting all the people who accidentally click on Threads as they are scrolling through their Instagram feed? I have literally never heard anyone bring up Threads in conversation, I never see it referenced on other platforms, I never see screenshots of the content there; why is this the case if so many people are using it?
Those numbers are a big legal thing because they influence share price. I don't know how exactly they're calculated and I'm sure the most positive spin is used (such as click through from instagram) but if they were actually fraudulent there'd be massive consequences. It's not like anyone is ever hesitant to sue Meta.