Most startup plans involve growing fast and big... which let's face it, never really happens until far too late for the whole project to survive.
|Virtual customers are better than too few real customers...|
However it's not necessary to wait for real customers before you can start gathering interesting feedback that makes sense. Why so? By making a prototype (or a Minimum Viable Product if your product doesn't exist yet) you'll be able to attract people who will act as virtual customers and provide feedback that can be very close to real customer's one.
Plan your prototype carefully enough to make people's feedback relevant but don't let it take too long because it'll prevent you from starting the feedback collection early. If all goes well, the virtual customers should turn into real ones once the product is out!