You cannot say a 20% drop AH is "expected" by investors. If this was truly "expected" then the slowing growth would've been long accounted for in the price.
*but you're not alone.
And to your point, investors could have sold for over 25% of profit as well if they had sold it earlier today. So I still stand by saying that this is huge change. Besides, this thread is not about long term, just invest in index funds so I just fail to see the point.
Stock prices should come with uncertainty bars, like scientific quantities, but there is of course no standard way to calculate and denote that.
The EMH would say that a company's stock price accurately reflects all current knowledge about a stock's underlying value. So from the point of view of an investor in publicly traded companies, a stock's price and its underlying value are the same thing.
Options have an expiration date because as the further you go out the more uncertain things are, and the harder it becomes to make a market in options in a way that (a) won't result in a high chance of losses and (b) will have someone actually willing to buy them. That's why you don't see 10, 20 or 50 year options.
That doesn't hurt their value for pricing uncertainty because as options expire, new ones are minted. Therefore, you can always use them to price the uncertainty of the price to the next quarter, the next 6 months or the next year.
Put another way, "don't get high on your own supply."
A 20% drop in 2 trading days has around a 1/700 chance of happening under a Black-Scholes model.
So yes, this does happen in the market, but it's a reasonably rare event.
(As another data point on how rare this was, $190 puts were being sold for $0.3/share. Those are now worth $17, a 50x return)
Sources:
A single day move of twenty percent is rare even in small caps. Facebook moving that much means the market did not except even the risk of something this big.
For comparison when Nissan announced a recall of over a million cars their stock moved less than three percent. Single day moves are rarely so large, and almost never this large for a single company.