Growth comes from two places

1. Your core audience starts to spread to word. You've built the network effect into your offering so that it works better when people tell their friends and colleagues. For example, Twitter and the ice bucket challenge.
2. Your success at serving this small but viable audience gives you the team, cash flow, and social proof to find a different set of customers.

The hard work of customer development is finding a reason for your customers to bring new customers.