The four cases in parts two to four are the deep ones. These three are short, sourced, and closer to what a consumer brand can copy next month. Each one pays in a different currency, and each one shows a trigger or a reward design worth lifting whole.

Hotmail: the footer

Hotmail launched on July 4, 1996. Its investor Tim Draper pushed the founders to add a line to the bottom of every email a user sent, telling the recipient they could get free email at Hotmail. The founders resisted, worried it would spoil people's messages, then agreed to a plain version.

In Adam Penenberg's telling, excerpted by TechCrunch in 2009, signups jumped to about 3,000 a day once the line ran. One email sent to India produced 100,000 users there within three weeks. Hotmail became the largest email provider in Sweden without spending on marketing there, while a competitor spent $20 million for far fewer users.

PublishedPenenberg, Viral Loop (2009), via TechCrunch, October 2009.

The trigger was the product: every email a user sent carried the invite. The currency was nothing at all; the product was free and the line cost nothing to print. A brand's version is anything that travels with the product to someone else: the packaging, the gift note, the order confirmation a customer forwards.

Harry's: the prelaunch referral race

Before launching in March 2013, Harry's put up a simple page: enter your email, get a personal link, and earn free products as friends sign up. Co-founder Jeff Raider wrote up the results on Tim Ferriss's blog in 2014. In one week they collected more than 100,000 email addresses, about 85,000 of them valid. About 77% came through referrals: roughly 20,000 people referred about 65,000 friends, more than three each.

PublishedJeff Raider on Tim Ferriss's blog, July 2014.

Three details made it work. The rewards were product, so they cost Harry's its cost of goods rather than cash. The tiers were visible, so every sharer could see the next rung. And the team blocked repeat signups from the same IP address and validated emails, the fraud brake from chapter 15. Raider also credited seeding it personally, sending it to friends and colleagues first rather than to the press.

A visible next rung turns a referral into a race.

Morning Brew: paid in status

Morning Brew, the business newsletter, launched its referral program in 2017. Tyler Denk, then its product lead and later a co-founder of the newsletter platform beehiiv, wrote that the company grew from 100,000 to 1.5 million subscribers in eighteen months with referrals as a main engine. Digiday reported that referrals drove about 35% of list growth.

PublishedDigiday, citing Denk. About 225,000 subscribers had referred at least one person and about 85,000 had reached three. Denk put the cost of a sticker reward at about 25 cents per acquired subscriber, against $2 to $5 across other channels.

Most of the ladder is status and access, the cheapest currencies in Pick the Currency. And the program counted a referral only after the new reader confirmed their subscription, the newsletter's version of completed value.

What the three share

Do this

• Write a four-rung ladder where the first two rungs cost you nothing but status or access.
• Find your footer: the one thing that already travels from your customer to someone else.