That's why they've stockpiled $105B and $55B in cash respectively, though. Facebook could run the company for 1.5 years if their revenue went to zero; Google could run it for 2 years. They know how the business works and position themselves to weather and profit from downturns.

Some weaker shareholders may sell if they're only interested in quarterly numbers, but in the long term their revenues go back up to what their userbase justifies.