Zynga went through its own private flash crash. The straight drop and rise and drop in its stock after Facebook’s IPO is the classic signature of program trading. Whatever algorithms traders were using agreed that the stock was hugely overvalued, then undervalued, then overvalued again. So they all screamed “SELL” or “BUY” at the same time. 

Then they stopped. Trading resumed at a more human pace. Why the change? One plausible explanation is that traders unloaded Zynga stock because now they could buy Facebook stock. They did so en masse — until Zynga stock fell so far that it became too cheap to resist. Then they bought it en masse — until Zynga stock rose so far that it became too expensive to resist dumping.