Interesting, looking at it from that side. How does it work with traditional real world ads, billboards for example. I'd expect them to have a similar challenge. Not whether the ad was put in front of people, but the high level impact of the whole campaign.
How would advertisers know whether OpenAI even applied the ad to the paid for number of people's conversations, I don't know.
This was posted in Show HN today [0]. Looks like queries just query ChatGPT themselves for whatever terms they want to track and then correlate revenue with that. But ChatGPT will probably do it like affiliate links, where there is a tracking parameter or cookie that identifies that it came from ChatGPT.
Then you move to the paid plan, then they move ads to the paid plan, then you move to the premium-extra plan, etc.