Megabrands like Dropbox, Instagram, Snapchat, and Airbnb were barely a blip on the radar years ago, but now they're worth billions—with hardly a dime spent on traditional marketing. No press releases, no TV commercials, no billboards. Instead, they relied on growth hacking to reach users and build their businesses.

Growth hackers have thrown out the old playbook and replaced it with tools that are testable, trackable, and scalable. They believe that products and businesses should be modified...