Gmail

First Google built a superior product. Then it built excitement by making it invite-only. And by steadily increasing the number of invites allowed to its existing user base, Gmail spread from person to person until it became the most popular, and in many ways the best, free e-mail service.

Mint did something that few startups had done before to increase awareness and build excitement for its launch: it asked people on its prelaunch waiting list to recommend Mint to their friends in return for priority product access.

Viral growth builds on the power of networks to acquire users, often free of charge. Product-Driven Viral Growth Viral growth is deeply misunderstood—you might read the phrase and think, is this the same thing that happens when a funny video “goes viral”? Or maybe it makes you think of an ad agency organizing a clever stunt to share on social media

A common way to drive revenue through email marketing is doing lifecycle campaigns aimed at upselling customers.

Groupon generates referrals by giving users an incentive to tell their friends about discounts.

the product must be inherently worth sharing—and then on top of that, you must facilitate and encourage the spreading you’d like to see by adding tools and campaigns that enable virality.

Sean Ellis, Dropbox built one of the most effective and most viral referral programs of the start-up world. It was as simple as placing a little “Get free space” button on the front page of the service.