The other big news was that Facebook was testing ads outside of their own platform. The possibility of taking on Google's Adwords and adding more than a few billion dollars in revenue should have increased the price?
You're layering a lot of your believe about Facebook onto that assumption - maybe people saw that news and concluded Facebook would fail in external ads and lose a bunch of money, which decreased the stock price. It's hardly unambiguously good news (like record profits or something).
