Well, the stock dropped a bit after the report, but then it fell off a cliff during the call. It must be the forecast and not the results that is really spooking people.
The guidance is nice. It resets expectation, especially amongst the analyst community. Makes it easier for FB to beat expectations the next time around.
How are stock price drops reported when the market is closed?
The same way they are reported when it's open: approximately, with the cooperation of the various market makers.
The closing auction signifies the end of regular trading hours (where more rules are in effect) but not the end of all trading. US stocks now trade 24/5 on at least one market, and well beyond 4pm ET on many more.