"If it paid well" externalizes the ills it inflicts on other people. And they matter, too.
Hurting others unnecessarily is the only sin in the world. Don't ever, ever be That Guy.
Nobody is 'hurting anyone' - not even slightly, by ensuring that their free product also has ads.
If ads are unscrupulous, or if the company is doing shady things otherwise, then yes - bad.
But there is no moral argument against making sure that decent ads work with a free product, or when ads are part of any product wherein the social contract is to that expectation.
Facebook has ads, just like CNN and Cosmopolitan, that's normal, ethical, and within the expectations for user's experience. Again, shady things notwithstanding.
In 2018, people can pay or see ads, or a combination of both, there is no pragmatic way around this, and too many decent products depend upon ads for their existence, that's where we are until someone comes up with something better.
You are correct in the idea that there are "decent ads"; they are not in any meaningful fraction, however, web-served ads. If you're familiar with the theory and the development of advertisement, you'll notice that what you won't see on the web in meaningful quantities are things like brand-anchoring advertisements ("brought to you by"), which I tend to think actually provide societal benefits in the way they are deployed; they provide some level of community participation on the part of the advertiser and they anchor the advertiser in the same firmament of society as the person receiving the advertisement.
What we instead have, and will continue to have and this is why advertising on the Web--as an aside, you can find oases of ethical advertising in places like podcasts, it'd be nice if the rest of the Web was like that!--is profoundly toxic and bad and should be killed, is an unending torrent of calls to action carefully designed and split-tested to claw maximal real estate inside the receiver's head. They amount to psychological assault. It's screaming at the receiver and for many people in the industrialized world the background radiation of this kind of advertisement starts when they wake up and continues until they go to sleep. And I think it's no stretch to assert that that's bad for the health of individuals exposed to them and it's pretty obviously, at Facebook scales and with Facebook morals, bad for society as a whole.
And yes, some people are fortunate enough to have the technical capability and the platform choice to escape some or even most of it. But choosing to shrug and blather about how you're happy to make this worse is a really bad look.