Sam Altman argues in How to Grow Huge that the only way for a startup to grow really large is to create products that people love and promote. As the user base grows, users attract ever larger numbers of users to the product, producing compounding growth. The point is a terrific one and I think it can be generalized. To grow really large, startups have to create proprietary distribution channels. The one Sam champions, word of mouth marketing intrinsic to a strong brand, is one example of a proprietary distribution channel.