That is not worth a 20% drop in stock price.
Yup, both the earnings and revenue misses were relatively small.
However, fb's forward revenue guidance was the shocker: ~10% drop next quarter, and another ~10% drop two quarters from now. If you add that up, fb is telling us that they are forecasting a 20% revenue drop in 6 months. That's as klunkish as the side-o'-beef being chucked into her scooby van on 9/11/16.
So the after-hours traders traded appropriately.