Referral

One customer sending another, on purpose. The mechanised cousin of word of mouth, and nearly as trusted.

A referral is word of mouth given a rail to run on: the recommender is offered a moment, a link, sometimes a reward, and the friend arrives pre-trusted. It is nearly as persuasive as the organic kind because the sender still stakes a sliver of reputation on it.

Nearly, because incentives dilute testimony: the reader discounts a recommendation exactly as much as it seems paid for. The design rule is to reward sharing without renting it, keep the incentive small enough that no one would recommend a thing they did not believe in for it. The product remains the engine; the programme is only the rail.