Paid services create a liability for the company to adhere to its terms, as the payments create a minimum floor for establishing damages in court.

A company could provide terms of service and perhaps an explicit creative works agreement defining the exact uses for which the company will use the data (to display on the website, to comply with legal requirements, etc).

It would still ultimately rely on a whistleblower or voluntary auditing, but the combination of clear damages and a not-open-ended terms of use would keep the company lawyers invested in keeping the company honest perhaps.

They probably can't, which is why I'm pessimistic about the idea being viable - unless there is some way for the application to be open-source and quasi-federated; for example with people/companies/communities hosting their own nodes - like Mastodon but with some kind of auto-discovery for regular users to make it approachable.