Well, to be fair many of them had some kind of prototyping to see if customers like the product or not before building the complete product and releasing it. AWS was first developed internally to solve their own developers problem. Dropbox had a very viral successful video to gauze customer interest. For Facebook, Mark Zuckerberg did develop many side projects (like gathering opinions about some historical art for class project etc) during his time in college before combining all the best features to release Facebook (which didn't have photo gallery feature at the time of release). One overlooked part I think is the network. I remember listening to interview of Stripe founder and he mentioned how that network helped in getting initial traction to the product, so it's not all about building in silos for months and hoping customers would come when the product is launched.