Surprised that this post does not talk about one of the most successful referral programs I have seen (way before Dropbox): Paypal. I certainly made a bunch of money and got a few friends signed up back in the day.
You see similar rewards here in UK for referring people to new bank accounts, Chase made a big move over here and was giving £20 to the referrer, and £20 to the referred for minimum commitment. If you got the money it's a fast way to gobble up market share.