The philosophy of Dropbox’s referral program was very plain. Since the product offered storage space in the cloud, they decided to reward people with more free space not only for referring their friends but also for accepting an invitation.
In order words, we’re talking about a 2-side referral program for a compelling product, that rewarded both sides for completing the desired task; registering for Dropbox.
The details that made a champion:
It was part of the onboarding process
People had a clear view of the benefits
Ridiculously easy invitation process
People knew their referral status
This is a tremendous case study with excellent detail. And as a bonus, you can find templates to use here: https://viral-loops.com/templates/refer-a-friend