The list of users who favorited a post can be accessed at /posts/[id]/favorites. e.g. https://e621.net/posts/2623630/favorites
However, it seems that there is no API endpoint for this info, as https://e621.net/posts/2623630/favorites.json returns "406 Not Acceptable"

Also, is there any chance of this being added to db_export? Taking into consideration that it would possibly expose favorites from users who recently enabled "privacy mode". (just read that it won't be done in another topic)
I am playing around with building a recommendation engine, and having access to this info in bulk would make collaborative filtering a trivial task, without having to hammer the API with requests.