Conspiracy theory time! I don't really believe either of these with any non-trivial degree of probability, but I figure at worst I'd get downvoted, and maybe I'd get some insightful takes. There was an opinion expressed after '08 that the crisis was used by "elites" to conveniently get rid of (as in fire, mostly) a bunch of people that were, to them, superfluous. Like, that was long overdue and there was a good excus…
People are going to lose jobs, only some of which will be replaced by big companies (since they're inherently better at scaling people's work as well as automating jobs), so we're going to be stuck with a bunch of unemployment. Unemployment and the economic uncertainty that comes with it will cause a substantial drop in consumer discretionary spending, which I believe makes up like 70% of spending in the economy.
If you're Amazon, maybe that's fine because to your point, if the pie shrinks significantly but their share of it goes up enormously, they may still benefit even in the short term. But what about Apple, which is hitting new highs? People need to buy stuff, and since Amazon has everything, even if the type of stuff they're buying changes, they'll still go there. But they can (and I suspect very much will) put off buying new high end luxury devices like iPhones/iPads/Macs. Similarly, businesses that would be buying those devices will probably lengthen their upgrade cycles as well.
Same story with Facebook - stock is doing great, but small businesses represent a significant portion of their income. Plus a huge amount of the advertising is for consumer discretionary stuff, and it's a very easy kind of advertising to dial back quickly when marketing dollars go down.