Good point, I'm not sure. I'd be interested if anyone knew a good estimate for how much of their operating expenses the ad-tech side took.
Presumably Facebook would abandon ads if there was more money to be made in paid subscription, so I'll use corporate greed as evidence that the math doesn't work out. But I don't personally know.
It would have a far, far smaller codebase, and only need 10% of the engineers and resources(servers, ram, databases, etc) it has now.