Every outgoing Hotmail email included the signature line PS: Get your free e-mail at Hotmail. This was one of the earliest examples of viral marketing.

Dropbox offered users additional free storage space in exchange for referring friends. This created a powerful incentive loop. The reward (free storage) was directly valuable to the user base.

Airbnb created a way for users to automatically cross-post their listings onto Craigslist, which had a much larger audience at the time. This growth hack tapped into an existing network where their target audience was already looking for accommodations.