Google and Facebook both have the problem that their business model is predicated on having thousands of small advertisers bidding on ads. That's what they leverage to generate profit. In any market that's dominated by a single entity that smaller companies won't compete with for ad space, both Google and Facebook lose the ability to make money.
When the majority of people who want to buy nail varnish bypass search entirely and go straight to Amazon, Google and Facebook have a huge problem. The same is true if developers go straight to Stack Overflow, people looking for rooms go straight to AirBNB, people moving around cities go straight to Uber, etc, then Google and Facebook will have no one to sell ad space to any more.
I'm not about to suggest Google and Facebook are doomed; they both have talent enough to think of ways to meet these challenges. I would however suggest that they are not too big to fail.