The formal name for K-factor : the average number of new users each user pulls in by inviting others, where anything above 1 produces self-sustaining exponential growth.
From 2010 to 2012 Pinterest gated signups behind invites, and each new user could invite roughly 10 friends with about a 4% conversion rate. The resulting coefficient was below 1 but still meaningfully amplified paid and organic acquisition. Pinterest grew from 10,000 to 12 million users in eighteen months by combining the loop with press and Facebook integrations.