Dropbox built one of the most effective and most viral referral programs of the start-up world. It was as simple as placing a little “Get free space” button on the front page of the service. The offer was that users would get five hundred megabytes of free space for every friend they invited and got to sign up. Almost immediately, sign-ups increased by roughly 60 percent and stayed at that level for months. With more than 2.8 million direct invites a month because of the program, it’s not hard to see why.
