[neovi]

There's that, and there's also the type of investors.

Since retail investors can trade after-hours, we can assume that the people responsible for the drop are either professionals or retail, which allows for irrational selling.

Also, the stock market is a market. So there could be institutional investors shorting bad plays from the individual investors and aggressively selling the stock (since after-hours doesn't have as many traders, it's a good bet to short until market opens tomorrow).

The only factors I personally see to determine Facebook's return on investment/share price are 1. $ per user, 2. potential user growth combining Facebook & WhatsApp, 3. time; which all seem pretty lackluster for the amount paid in the acquisition.