The stats I see for Facebook are $70 per US/Canadian user in ad revenue. I'm not sure how much people would be willing to pay for an ad free Facebook, but it must be below $70 on average. And as the parent comment said, the users who would pay that are likely worth much more than the average user to the advertisers.

For the users who refuse to see ads, they'd either use a different platform or run an ad blocker (especially using the website vs the app).