There might also be considerable weakness in the metrics. I don't really have the patience to dig deep, but i wouldn't be surprised if parents create accounts as a way to easily monitor their kids activity etc and I don't know if that shows up in the stats or if there is a hurdle to being a "user". I don't use tiktok, but it's also possible some folks just use it as a messenger service if there is one, I know I don't use Facebook itself in any material way but do use the messaging feature on my phone. I also have some other SM accounts, but largely just to reserve my name lol, so I don't have impersonators or if I ever did use it having some silly name.
Then the other thing would be how many people have multiple accounts, and how they are accounted for. I know some crazy people that create alternate SM account(s?) to creep on other people. Apologize if the R word is offensive, but I think this is largely right, though TikTok should be added to the FB.