This, for me, is the weak point in an otherwise excellent article. A lot of investments, valuations and jobs rest on this assumption. Facebook's PE ratio is currently 127, this assumption is the reason that it isn't around the same level as other entertainment companies like, say, Disney (17) or News Corp (56). And when Facebook's valuation rises like that and people invest at that level then there's a whole lot of money for paying engineers >$100k salaries and buying up pre-revenue businesses like Instagram. So, even if you're Google and you're bringing in real money, the application of this assumption to a few big cases permeates through the whole system and means that you too have to pay engineers >$100k and you too have to pay more to get hold of someone like Nik (makers of Snapseed).
PG logic appears flawless, but as a seed fund manager in the middle of this ecosystem, he's working several layers of abstraction up from some big applications of this assumption. So much so that it probably doesn't feel like an assumption to him. After all, he didn't value Facebook[1] at that level.
I genuinely hope this assumption is correct, because a lot of people and livelihoods are depending on it.
[1] I'm using Facebook here as an exemplar, I'm sure there are lots of other companies out there with valuations that are due in part to the assumption that users = revenue. My argument is that when someone sets a valuation based on this assumption it has a knock on effect to the whole ecosystem.