Thank you for the compliments :) I used 50-100 datacenter proxies. I just logged requests made by the iOS app with Charles and then recreated the headers to the best of my ability though the server did not seem to be very strict at all. Worth noting though that static residential proxies are not too expensive these days anyways.

Re the API: The model does actually run fairly well on CPU so it probably wouldn't be too expensive to serve. I guess if there is demand for it I could do it. I think most social book sites would probably like to own their recommendation system though.