Let me repeat for you: these valuations are not above criticism. I just want to see better reasoning involved than literally balking at big numbers. It comes across as the same hysteria that drives the valuations up in the first place, just in the opposite direction.
It's really not a lot to ask to simply elevate the caliber of discussion. For example, I could respond to your second paragraph by stating that it is very difficult to claim what an objectively good valuation is without a understanding of what Airbnb's future plans are. Expedia's market cap is on the order of 17B. We have incomplete insight into Airbnb's plans for future markets.
See? That isn't so difficult. I'm not blindly attacking or defending the valuation, I'm reasoning about it instead of saying, "These numbers are big so I can't see how they possibly need them!"
In reality, opinions from anonymous engineers who know what is being sold may be the best you can get.
Clearly the market is different now, but basic economic laws of nature don't get repealed. The reason a valuation question is the top comment on all these stories is because the valuations appear, objectively, to have no basis in a realistic assessment of the future discounted cash flows of the business in question.