Taking a note from Netflix, Disney+ has issued a press release about the impressive performance of its latest exclusive release without including any numbers or even notes on the methodology used. One way or another, The Falcon and The Winter Soldier apparently rated as "the most watched series premiere ever on Disney+ during its opening weekend...and the most watched title overall for the same time period on a global basis."
Unsurprisingly, premieres for WandaVision and The Mandalorian season two are the other highest-ranking debuts based on whatever system Disney is using, but it seems that HBO Max's Snyder Cut of Justice League didn't manage to grim-dark everyone into ignoring the MCU for an entire weekend.
While streaming services carefully collect data on what people watch, when they watch it and even where they hit stop to switch to something else, they're not into sharing that information with the public. Third-parties like Nielsen provide some insight into what they think is popular, but otherwise we're largely in the dark. In early 2020 Netflix explained a change to how its ratings are calculated, and it started releasing more detailed viewer numbers based on that method, but it appears Disney+ isn't going to go that route.