Or basically one Adobe. Which is pretty wild. But honestly, this is a short-term mis-pricing and over-reaction. Might as well think of this as "Facebook stock on sale, 20% off!". But since Facebook makes up such a big percentage of an S&P 500, simply load up on $SPY, $VFINX, or one of your other ETFs/index funds and buy on the dip without having to risk over-exposure to one stock.
It's now at what the price was at 3 months ago. A lot to lose in one day though.
It looks like a particularly large investor just fat-fingered the sell button.