Feedback loop: Creative Market tells you when someone signed up because of you (referral link), they also let you know how far away you are from reaching the next level of referral bonuses. 

p.s. They’re opening on Thursday until then you can get 5$ for free by signing up.

Dropbox referrals: as far as I know Dropbox has been one of the earliest heavy users of great referral bonuses. Let’s take another look. Here’s a view of all the (official) ways to top up your account, on top you can upgrade right away.