For the biggest ones: How many of those active users are bots, advertisers, and scammers? I'd guess about half on Facebook.
Also, is it considered "active" if you have a dormant account but have the app installed on your phone and it still watches what you're doing? What if you only use it to communicate with family because it's the only internet they understand?
Further, what about duplicate accounts or "secretive" secondary accounts so you can click on the depraved stuff you like without that showing in your public feed?
I feel like the real numbers for the big ones are massively inflated by issues like these.
The Fediverse is small enough to as of yet not be affected. Once it gets large enough, it will have all of this, too.