[cauterized]

Big fish in small ponds frequently (to stretch an analogy painfully thin) start to take over nearby larger ponds once they've dominated their niche.

A few examples that come to mind include:

Amazon (books -> retail in general -> streaming media, hardware, IAAS...) Google (search -> ads -> mail, calendars, etc. -> hardware and operating systems...) Facebook (social networking at Harvard -> social networking in the Ivies -> social networking at colleges -> social networking everywhere -> messaging, ads...) Zappos (shoes -> handbags -> clothing, jewelry, sunglasses...)

And those first three grew not just by taking over existing markets, but by becoming popular enough that they enlarged their own markets. Before Facebook became big, social media just wasn't something the vast vast majority of people over fifty had any involvement in. Now everyone I know (including two over-90-year-old great-grandmothers) has an account.