The Importance of Unit Economics  

If you hold yourself to the standard of making a product that is so good people spontaneously recommend it to their friends, and you have an easy-to-understand business model where you make more than you spend on each user, and it gets better not worse as you get bigger, you may not look like some of hottest companies of today, but you’ll look a lot like Google and Facebook.

— Sam Altman, YCombinator