What does FB sell?
Facebook sells information about it's users that they have been more than happy to submit in various forms. FB and other companies collate that information with information they acquire from other sources. What would keep them from matching up nude photos with individuals using facial recognition/body recognition? You don't even have to have an account if some horrible person you thought of as a friend posted a picture of you and tagged it with your name. FB has a nasty habit of turning private things public without notice and even sometimes by accident. They might even "accidentally" make the nude photos available to their customers (not users, people that give them money for user information) due to a misconfigured flag or something. After noticing that bandwidth is off the hook, they might plug the leak, but the valley below will already be flooded.