The short answer
Ritesh Arora and Nakul Aggarwal built BrowserStack for a pain they had felt themselves, testing sites across browser versions, and their first distribution move was talking to developers where they were already complaining: Twitter, Stack Overflow and forums. The turning point was jQuery's John Resig tweeting about it, which brought roughly 10,000 beta users in three weeks. From there it compounded: about 20,000 dollars in monthly recurring revenue within four to five months, a million in annual recurring revenue in year one, and 20 million in four years with only about 50 people. Arora is clear that this was driven by word of mouth and influential developers talking about the product, not by a marketing machine. The precondition was a product so obviously useful that a respected developer wanted to tell people about it.