We can considered Dropbox's referral program, which significantly boosted user acquisition, and Airbnb's Craigslist integration, which rapidly expanded their user base.

The Tinder story is a fun one.. it's in Andrew Chen's book Cold Start. They basically brute-forced the first few thousand users. There's more to it, but they threw dope parties on college campuses and made everyone install the app to get in.

Growth hacking that I always think about is when a product is used to improve itself. So a cold email might be used to find leads for the business, etc