If you've managed to gain traction with a service that has a clear business model (Twitter and Facebook would obviously qualify) then you are on the right track. Booking revenues is good but mostly because it demonstrates that you can book revenues, not because you actually need the cash.
Isn't that the point of those companies though, that you aren't seeing traction for a while? It could be that PG is making the distinction implicitly between revenue/profit and traction.
So for example twitter, FB etc... were not profitable or getting revenue well after they had already amassed millions of users. If that is the case, then we aren't learning anything new and it doesn't help people who are trying to make new, hard, breakthrough technologies.