Worse, Facebook makes a lot of mobile advertising money from app install ads. If startup investors cut back the flow of money, that advertising revenue dries up. There will be a glut of unemployed developers just as Facebook starts reporting declining revenues. Facebook will need to start reducing costs, not hiring, and definitely no acquihires (startups with viable business models might be a different story).
Google might clean up with their pick of the best surviving startups and the best devs going cheap, because they have more diverse sources of advertising revenue, but they might also be under pressure to control costs if investors panic and flee tech.
That Facebook makes money from mobile advertising is a good point: In the dot com era, many dot coms made money very briefly selling ads or services to other dot coms, who were willing to pay way too much. Now many companies are valuable because other companies will pay too much for user acquisition. If they stopped, their would be repercussions for a lot of companies in the middle (not Facebook) who see their business model go away.
If the base of the value chain isn't making money (and I think honestly in the current case even the middle isn't) the whole ecosystem will probably contract.
Google might clean up with their pick of the best surviving startups and the best devs going cheap, because they have more diverse sources of advertising revenue, but they might also be under pressure to control costs if investors panic and flee tech.