I'd say, no you had no shared customers. Yelp had the customers, and you wrote some app that uses Yelp. For no real reason you assumed that you could be part of them (as with Facebook earlier). Sure you had some kind of informal permission at first. And then they revoked it. That's their right to do, right? Maybe you should not write Facebook or Yelp apps, if you are neither Facebook nor Yelp nor asked by them to write clients for their walled gardens. Or if you really want, set up an actual license agreement with them. Those networks are walled gardens and they don't want to cooperate with you that way. They are just not those kinds of shops which you can cuddle with. If you want to create something that has an own value, create something more than just another Faceyelptwitterddit frontend.
The good thing: You took your lesson. Maybe this time it was sufficient to actually learn sth.
Person who paid for my app also later uses the Yelp website via a link for a specific review from my app. Also a customer.
Therefore shared customers.
> Sure you had some kind of informal permission at first.
I don't know why you would define it as informal. They had an official review process for deciding what apps to grant higher daily API limits. My app went through that review process 10 years ago including considering its functionality, screenshots, and I believe I even sent them a prototype back then. We had a few emails back and forth to confirm my intentions to only develop for the Mac and what I was building.
I'm sure that got lost in the corporate shuffle over 10 years but I clearly had their permission to build the app and in fact given the relatively high API limit they gave (25000 per day versus the free at that time 10 years ago I think being 1000 per month), arguably blessing.
You created this account solely to respond on this post and all of your replies are defending an ostensibly horrible business practice, which is to give your API users less than one business day of notice to switch to a different tier.
No matter how you cut it, that is simply terrible developer relations/customer service and a terrible way to run a business.
The good thing: You took your lesson. Maybe this time it was sufficient to actually learn sth.