> Most sites are not in business of running ad marketplace.

You don't have to run a marketplace. Back then, people were calling via phone to put an ad into the newspaper. The same concept can still apply today.

You can just place an ad _where you actually want it_ instead of having a shitty decision engine decide it for you. Marketing budgets are messed up all over the place since social media, because they're really inefficient in finding the customers that actually wants to buy their stuff.

If you don't believe me, just take a look at facebook, they're trying to shove ads up your feed 99% of the time. It's a feed of ads, not a feed of "friends" anymore. Good luck trying to catch the 1% of posts that were actually the reason why you used the platform in the first place.