Viral Loops & Referral Mechanics
Part of the Product Management Roadmap
In short
The cheapest user is one your existing users bring in. Referral programmes and shareable features turn happy users into a growth channel.
Designing a product so existing users naturally bring in new ones — through sharing, invites, or simply the product being visible to others (e.g. a Calendly link, a Zoom invite).
Think of it like this
The momo stall offer
"Bring a friend, both get a free plate" at a busy momo stall. You bring Rohan, he loves it and brings his roommate. Each happy customer brings more. The stall barely advertises, because customers do it.
Watch it happen
The viral coefficient, K
1/5K measures how many new users each user brings in through invites.
The big ideas
What you need to know
Two-sided rewards
Rewarding both the inviter and the friend often works better than rewarding one side.
Ask at a happy moment
Prompt referrals right after a win, like a delivered order or a finished course.
Built-in virality
Some products spread just by being used, like a shared payment request or an invite to a document.
Watch for fraud
Rewards attract fake accounts. Set limits and reward real activity, not just signups.
Save yourself the pain
Common mistakes
Oops
Paying for every signup from a referral.
Do this instead
Reward after a real action, like a first order, to avoid fake accounts.
Oops
Hiding the referral page three menus deep.
Do this instead
Show it at moments of delight, and make sharing one tap.
Your mission
Calculate K for a campaign
Quick check
Users send 5 invites each and 10% of invites sign up. What's K?
Go deeper
- 1Study the mechanics of a referral program you've personally used or seen.
- 2Calculate (roughly) what a 'viral coefficient' greater than 1 would mean for growth.
- 3Design a lightweight referral mechanic for a product idea of your own.
More topics at this stop
Growth Loops vs Funnels
A/B Testing Fundamentals
Writing a Testable Hypothesis
Statistical Significance in Experiments
Feature Flags
Experiment Design & Sample Size
The Hook Model
Onboarding Optimization
Activation & Retention Levers
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