Referral programs

Referrals are a designed viral loop: explicit incentives for inviter and/or invitee. Dropbox's classic two-sided reward (extra space for both) is the canonical case. The reward was in-product currency aligned with the value metric, the ask came after the user felt value, and the loop was easy to understand.

Collaboration virality (must invite to get value) has better intent and lower fraud but only fits multiplayer products. Referral bonuses work for single-player tools but can attract junk signups.

Walking through the checklist

1. Who to ask: activated users, not brand-new signups.
2. What to reward: something that increases usage (credits, storage, seats) over pure cash when possible — cash attracts referral professionals and low-quality signups.
3. Two-sided vs one-sided: two-sided usually converts better; one-sided can be enough if the product is already highly shareable.
4. Fraud and gaming: self-referrals, disposable emails, incentive stacking. Budget engineering time for this or the loop's unit economics lie.
5. Measurement: activation and retention of referred users, plus K on activated users.

Engineers own the mechanics: signed invite tokens with expiry, rate limits to blunt referral fraud, Open Graph previews that raise invite CTR, and idempotent referral credit ledgers so double-grants do not bankrupt a Dropbox-style reward.

Instrument the edges:

activated_users
  → % who hit invite affordance
    → invites_sent / activated_user   (i)
      → invite_link_ctr
        → signup_rate from invite     (feeds c)
          → activation_rate of invitees
            → their invites…          (next turn)

In interviews, draw the loop, label the rates, estimate K and cycle time with rough numbers, and say what you would instrument first.

Common mistakes

• Asking at signup. The user has not felt value; invites will be junk.
• Cash to anyone. Professionals and throwaway accounts show up.
• Counting signups, not retained referred users. Week-1 K can hide week-4 emptiness.
• No fraud budget. Self-referrals make the unit economics a lie.

Check yourself

When do you show the referral ask?

After activation — once the user has felt value — not on the signup confirmation screen.

Why did Dropbox give space, not cash?

Space is in-product currency aligned with the value metric. It increases usage instead of attracting bounty hunters.

Key takeaways

• Ask activated users. Reward in-product currency when you can. Prefer two-sided rewards.
• Measure referred activation and retention, not signup count.
• Budget for fraud: tokens, rate limits, idempotent credit ledgers.

Further learning

Back to the map in growth loops.