I don't think Dropbox "pioneered the model of the double-loop referral." Banks and phone companies have been doing that for decades. It's a good idea.

But Dropbox should get credit for cleverly using referrals to give people more of a thing they want (storage space) rather then reducing a thing they don't want (cost). They were kind of forced into this when they onboarded tons of people into a free plan, so they couldn't reduce cost, but it's still clever.