Dropbox made this technique popular by rewarding you with more storage when you refer someone, while at the same time giving that new referred user more storage as well.

The genius in this particular example is that it doesn’t cost them any revenue. Whereas most double loop referral programs offer to give users a discount and then give a discount to the new referred users. This is still a good option, but does your product/app offer something else that users will find of value?