Facebook ads were the cheapest for me ten years ago.
We were marketing a product that many people were happy to know existed. The dashboard gave us tools to really delve into demographics. Of all the ridiculous personal data Facebook collected, the best demographic filter was allowing me to narrow in on pages someone liked or interacted with. We were selling things related to cruising sailboats, and we could target an audience within 30 miles of Fort Lauderdale who also liked Sailing Magazine. Moreover, we could use a pixel so that only people who had also visited our website saw the ads.
Facebook had a policy of rewarding high-quality ad content. If people clicked the ad, or better yet left positive comments and discussion or shared, the price drastically decreased to fractions of a cent per impression and click-through. We were able to get ads shared a lot with people tagging other people about the product suggesting they might be interested in it. That was the holy grail for copy that we always strived for.
Of course, they got rid of all that. But at the time, it was a great way to target an audience based on third-party pages they liked, giving them ad content about products they were generally interested in—and products they were happy to know they could purchase because they had value.
Ads configuration is like gambling in Las Vegas, in that the easier the game, the worse the odds—like slot machines—and the more the player has to interact, like Blackjack, the better the payout. When done well with good configuration, we were getting 1000s of click-throughs for dollars. It was amazing.
The point is that Facebook rewarded ads that people positively interacted with, as it meant the quality of the news feed wasn't hurt by the ad.
There was a time when ads benefitted everyone, the buyer, the seller, and Facebook.