According to Dropbox: "You can get extra space by inviting your friends to try out Dropbox. If a friend uses your invitation to sign up for an account you will receive bonus space." This means that Dropbox uses a method for providing a customizable reward service that include monitoring user activity associated with a web site, comparing user activity to conditional rules and determining whether to generate a reward from an operator rewards based on satisfaction of the conditional rules.
This application received a non final rejection as being anticipated by On Line Loyalty Program US2006/0074755 . It is probably more on target than Dropbox. The application you mention also had issues with non eligible subject matter and vagueness. A response to that rejection was submitted in March. This all can be seen at USPTO's public PAIR.