It's not clear to me that the main hypothesis of this engine is correct - namely, that request recipients who are closely clustered to other users are more likely to accept the request. Do you have data that supports this hypothesis?

That said - request recipient optimization works, and offering it as a service could be a successful business. I think your primary value proposition (higher CTR's on lower volume) is solid, but would like to see verification that your approach (using clustering) is valid for that use case.