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How a one-line email footer built a 12-million-user company
💡 Viral growth hinges on the k-factor — invites per user times conversion rate — and only compounds when that number stays above 1.
In 1996, Hotmail was a tiny startup fighting for attention. Then investor Tim Draper suggested adding one line to the bottom of every email users sent: "PS I love you. Get your free email at Hotmail." Every message a user sent became an ad, delivered by a trusted friend. Hotmail reached roughly 12 million users in about 18 months — and Microsoft bought it for around $400 million.
The engine behind this is the viral coefficient (or k-factor): the number of new users each existing user generates. It's simply invites sent per user × the share of invites that convert. If every user brings in 1.2 new users who each do the same, growth compounds on itself with no ad spend.
The magic number is k = 1. Below it, each generation of users is smaller than the last and growth fizzles. Above it, growth is exponential. Dropbox later ran the same play: its two-sided referral program gave free storage to both the inviter and the invitee, permanently lifting signups by roughly 60% and turning users into a distribution channel.