Meta's Threads has had its daily users down 20% since Saturday and user time spent on the platform is down 50% ~ CNBC

Companies like Similar Web use web traffic data to measure what's going on with a website.

It's kinda like standing outside a store and counting the number of people going in, and timing how long they stay. You can't know what they're doing inside the store, but you can deduce a lot about the business.