Can someone explain why FB can't become a paid service that engineers itself for the good of users?

I don't know anything about corporate financing but naive napkin math says if their 2 billion user base pays $1/month they would be pulling $24 billion a year. Wikipedia says their revenue is $24b/year. Now if they removed their entire advertising aspect (the human labor, infrastructure) their operating costs would go down and that $24b/year would mean more than it does now.

Is that bad math? I would readily pay $1/month to use a service designed for our well being rather than advertisers. I think anyone would. Why isn't this a thing?