Hotmail

When Sabeer Bhatia and Jack Smith launched Hotmail in 1996, one of the first free web-based email services, they had almost no marketing budget to compete with the internet giants of the era. What they had was a small, deceptively simple idea a venture investor reportedly suggested: add a single line to the bottom of every email sent from a Hotmail account — "Get your free email at Hotmail" — with a link back to sign up.

Every email a Hotmail user sent became, without any extra effort, a small advertisement to whoever received it. Every person who signed up because of that message became another person whose outgoing emails carried the same message to their own contacts. The growth compounded on itself, spreading through existing networks of friends, family, and colleagues rather than through any paid channel at all.

The results were extraordinary for the era: Hotmail reportedly reached a million users within about six months of launch, and around 12 million within eighteen months, at a time when getting online at all was still a meaningfully harder, less common thing to do than it is now. Microsoft acquired Hotmail in 1997 for hundreds of millions of dollars.

The mechanism later got a name — the viral loop — and became a deliberate strategy that countless products since have tried to replicate, with mixed success, because the trick isn't really the tagline. It's building a product where using it, by default, naturally exposes it to new people, so that growth doesn't require a separate marketing motion bolted on afterward. Hotmail didn't add growth to the product. Growth was built into the product being used at all.