In these cases, sign-ups or GMV can sometimes suffice if no revenue is being generated yet. I have a couple of stories of start-up spending too much time on sign-ups and shifting to revenue too late in the game. Scribd was an early YC company that went through in summer 2006. They spent their first four years primarily growing a free product because they were afraid of losing their millions of customers if they started charging. In mid-2010, they started charging and while they did lose over 90% of their customers, their revenue grew by infinity percent. This talk helps at least one of you get to product market fit faster.