The core challenge behind most new ideas isn’t funding—it’s sales. Most companies think they know what is involved to get sales. But if getting sales were that quantifiable, any rational company would simply do the math and pay its way through to linear growth or beyond. You can calculate the cost to drive traffic—maybe. With some experimentation, you can figure out what ads cost and then measure how many users click through to your site. But then what? How many buy? How many come back later and then buy? And even if you can, and do, measure all of that, you are still left with the question of why. Why are users buying or not buying? Is it the UI? Your product? What most companies punt to is the hope that their product will go viral. To be sure, a lot of companies do go viral—and this concept extends beyond the YouTube video that gets a million views. Companies like Box.net or Dropbox attract large numbers of users by having a great product that people want. Friends tell other friends, and all of a sudden the cost of acquiring a customer drops to a very low number.