I doubt few of the FB users I know wouldn't part with $3 in exchange for their network/contacts that FB provides.
Average revenue per user in the US/Canada is $26.76 [0] per user per year. Globally, the average is over $6.
I'd wager a bet that the ARPU for users willing to pay $30/yr for no ads is even greater than $27 - likely above $100 - because the only people who would pay extra to remove ads are people with disposable income (which are usually the exact people advertisers want to target when trying to sell stuff).
[0] https://www.cnbc.com/2018/01/31/facebook-earnings-q4-2017-ar...
You may balk at Apple’s high prices but at least you’ve chosen not to pay.
Also, maybe the costs of running facebook's infrastructure and code drop (and employee headcount, etc) off dramatically if they take an axe to all the parts of it that are devoted to advertising and user data tracking/sales and just focus on the useful core tech bits, which also helps increase the profit margin on the new income model. It's possible there's a rational price that the masses would pay that would still turn them a profit under this kind of setup.
If they charged $3 per month they would be making less money than they are now, even if nobody quit because it's no longer free.