This is a highly effective hack and though Dropbox did not invent this method they surely are one of the most famous examples of this hack, as they successfully increased their sign ups by 60% by using this simple hack.
Anyone who signed up for Dropbox got an option to increase their storage space by inviting their friends to try the product.
The minute the referred friend signed up for Dropbox, the original user received more storage.
But what were the incentives for the referred friend to sign up?
Dropbox offered extra cloud storage to the referred friend too if they signed up using the link given by the original user.
As mentioned, this increased the sign ups dramatically and generated viral demand for the links among friend circles.