Viral Loop
A repeatable process in which existing users help attract users who can repeat the process.
A viral loop occurs when product use creates an invitation, shared output, or recommendation that brings in another user who can repeat the behavior. The loop links acquisition to an existing user action. It can be deliberate, such as inviting collaborators, or emerge from the natural visibility of useful work.
Example: A collaboration SaaS user invites a colleague to comment on a document. The colleague joins, creates a document of their own, and invites another collaborator. When that sequence repeats, product activity generates new acquisition opportunities without requiring a separate advertising impression for every participant.
Watch out: An invitation mechanism is not proof of self-sustaining virality. Many invitees already belong to the same account, never activate, or do not invite anyone else. Define the population and observation period, and distinguish new users from new paying accounts. Incentives can increase invitations while reducing quality. A loop can be useful even below self-sustaining growth, but its contribution should be measured rather than assumed.
Further reading: Amplitude: getting started with PLG.
Related: referral marketing, PLG.