How many new users each existing user pulls in through invites or shares, calculated as invites sent per user times the conversion rate on those invites.
When Hotmail launched in July 1996, founders Sabeer Bhatia and Jack Smith appended "PS: I love you. Get your free email at Hotmail" to every outgoing message. Each user effectively invited every recipient they emailed, and Hotmail hit 12 million accounts in 18 months. The episode is the textbook origin of the K-factor concept in consumer software.