From Peter Thiel's book Zero to One: "At PayPal, our initial user base was 24 people, all of whom worked at PayPal. Acquiring customers through banner advertising proved too expensive. However, by directly paying people to sign up and then paying them more to refer friends, we achieved extraordinary growth. This strategy cost us $ 20 per customer, but it also led to 7% daily growth, which meant that our user base nearly doubled every 10 days. After four or five months, we had hundreds of thousands of users and a viable opportunity to build a great company by servicing money transfers for small fees that ended up greatly exceeding our customer acquisition cost."
Thiel, Peter; Masters, Blake (2014-09-16). Zero to One: Notes on Startups, or How to Build the Future (p. 137). Crown Publishing Group. Kindle Edition.