Facebook's biggest asset is their data. My guess is that if they can't show you ads, they lose some of the ability to track you and your behavior in terms of interaction with the ads, interaction which creates additional valuable data. Due to network effects / Metcalfe's Law, if you eliminate ads for X percent of the user base, this will have a disproportionately larger (downward) effect on the value of their data in aggregate. Which may not be totally offset by whatever revenue you can collect from users directly in premium subscriptions. I would bet they have surely done the math on this.
It would be a mistake to offer a paid version of either the search engine or the traditional social network.
Both are more lucrative than a rate at which users would be willing to pay.
US + Canada ad revenue for Facebook will be around $15.x to $16 billion for 2018. That's $64 per year per user (MAU). That is likely to continue to climb over time, almost guaranteed to reach toward $100/year. How would they possibly convince people to spend $10 per month to hide ads on Facebook? It would never happen, it's not worth it. It's nowhere near a large enough inconvenience for ... 99.9% of global users. You'd get a tiny set of users to pay; the time / engineering / upkeep / support / payment handling etc. for all of that, wouldn't come close to justifying what they'd get in return. The best case scenario is they break-even on the offering, vs what they'll be yielding on their monthly actives with advertising a few years out; a more likely bad case scenario, they can only command $3 or $5 per month for the 'feature,' and it becomes a big losing proposition instead.
Also, reducing their display potential / ad volume, is a real negative financial event for their overall business model, for both Google search and Facebook. They benefit in numerous ways from having more display potential, rather than less (not least of which, those advertisers will spend that money on alternative digital venues instead, building up competitors).
YouTube ads are interruptive, as with traditional tv ads. Facebook ads / promoted x are overwhelmingly text and sometimes with an image. You can scroll right on by the Facebook ads and continue with your use of the social product.
Facebook Watch, assuming it continues to grow into a legitimate YouTube competitor, may offer Facebook a reason to launch a paid subscription system however. People may pay to avoid the interruptive video advertising.
