The value of the data gets tricky fast. If they made everyone pay $10 for the service, they would make less money. They would make less money because fewer people would use the service. To offer it for free means more people would use it. While each person may provide less than the subscriber, the masses of the free users makes more money than the subscriber.
To profit more from free users, you just have to have enough people willing to use it for free, and wouldn’t pay for it, to where their revenue exceeds that of the $10 plus the added cost to run the service with more users using it.
Just for easy numbers, let’s say a free users makes Meta $1.00 a month. If there is a group of 20 people who use meta, and only 1 of those 20 people is willing to pay $10, then the paid service would make them $10 where the unpaid service would make them $19. Obviously super simplified math, but honestly the number of people that would pay for Facebook is probably a lot less than 1 in 20.
I am not saying $10 is a fair price, but rather it’s not a simple task to pick a fair price. Not that meta wants a fair price anyway.