Product metrics and goals
Viral coefficient
Also called K-factor, Virality
How many new users each existing user brings in. Above 1, growth feeds itself; most products are well below.
How it's worked out
Invites sent per user × share of invites that turn into sign-ups
The coefficient is borrowed from epidemiology's reproduction number. A value of 0.3 still helps: every 100 users you win bring 30 more for free.
How fast invites turn into users (the cycle time) matters as much as the coefficient itself.
In practice
Users send 0.07 invites each and a quarter of invites become sign-ups: the coefficient is about 0.017.
Common mistake
Counting users who would have joined anyway as referred. Track which sign-ups came from an invite.