the question becomes who chooses what to advertise, and who benefits from the advertising.

Yes this is the tricky part I think. If the apps choose to go it alone, they will decide. Or, a new team forms with a browser extension and audits/decides on which businesses can advertise, or no-one policies it...

If advertisers have access to the blockchain, they are going to want to try to present the right ads to the right people, or else it's a waste of time. Perhaps we, as the consumer, can just hide/ignore/flag the ads/businesses.

Early stages, but an interesting area to discuss, thanks for all the points raised!