(I'm not being sarcastic, just want to understand)
what's the point in saying "OK perfect interview" and then discarding without giving feedback? why would I risk applying again if I do not know where I failed?
To avoid an emotional confrontation with the candidate. 95% don't meet the bar, but they would be offended if they were told how they really perform. Some may even sue back if they happen to be in some minority group. Companies like FB don't have a shortage of very qualified candidates, so you not applying again is just irrelevant to them. The sign of a perfect interview is when the interviewer realizes that you know more than him.
What is a perfect interview from Facebook's perspective? Probably a large part of it is determining whether or not to move forward. For a person who wants to work at Facebook, a perfect interview is mostly one that's passed.