Yea, classic case of low vs. high margin businesses. Travel is a great example. You would think sites like Expedia would make a healthy margin on airline tickets. Due to the way it's set up (booking systems, airlines, competition) the actual net per ticket is something like $5 for domestic flights. Last I checked Expedia specifically, most of their revenue was on hotels.

Contrast that with a high margin business like consulting services or a proprietary technology/product and there's a huge difference in net profit between the two at $10m annual revenue.

Revenue is King, margin is Queen.

Does PayPal count money paid through its app as revenue?

Good point. I believe it (and similarly Amazon marketplace sales) uses nominee accounts.

I don't use Uber so I don't know if the payments are always to the same 'Uber Ltd' or not, but you're right that would seem to be a good option.

No. Neither does a bank. That’s, for lack of a better term, escrow.