What is your favourite growth hack story?

For me, its AirBnB's cross listing integration with Craigslist. Through a single button, AirBnB let its users cross post the same listing on Craigslist as well. Total win for the user - single submit two listings. When anyone searched for a rental on Craigslist, they would see a slew of AirBnB listings. Win for AirBnB - as it was now riding on Craigslist's popularity. Absolute genius! What is your favourite growth hack story?