If you're willing to do whatever it takes, you can probably round up a group of early users (unless your idea is terrible). I think few founders really doubt their ability to do this. What the founders are really looking for is some validation that the users will eventually grow to a large number. But if the mentality is to compare your early startup with the early version of Pinterest, it will be very hard to find any evidence that yours won't find similar success. A startup is thus an inevitable leap of faith.

In a way, this essay deflates the concept of 'proof' in the lean startup mentality. There is no way to prove that an idea will be a good one (although you can probably filter out particularly bad ones).