Google uses my watching data in YouTube premium paying or not. They only claim not to show ads , that’s what most users care about . Even if FB sells / uses the data , as long as you don’t see ads and promoted content there are enough people who will pay for it

20 million for YT @ 5$/month is more than 1 billion in revenue per year. Given that 20M is just 1% of the user base , there is probably not a lot impact on the ad revenue either.