One of the most powerful user acquisition channels for a service is its existing user base. People tend to listen to recommendations from friends which is why word of mouth is so powerful. Sometimes though, users need a little nudge to start telling their friends about how great a service is. That is where incentivized invites come in.

Dropbox’s invite prompt

There are a lot of services out there that use incentivized invites, but Dropbox has one of the best implementations I’ve seen. Whenever you login to Dropbox’s web interface, there is a constant reminder to invite more friends. Sitting in the top toolbar you can’t help but notice the little giftbox icon with the enticing label “Get free space!” rather than the more boring”Invite friends”. After clicking the giftbox Dropbox informs you that can get 500MB of space for each friend you invite. 500MB is a pretty sizeable amount, especially when you consider it is 10% of what you start off with in the free version of Dropbox. There are two important traits that an incentive should have and Dropbox nailed both of them. First, an incentive needs to be very appealing to the user. Badges, unlocking features, etc., only appeal to a subset of users. A good incentive is something that all users will find appealing. The second trait is that the user should start seeing the benefit after the first few invites. The average user will send invites out to only 2 to 3 people, so if it takes 5 or 10 invites to start seeing a benefit, then many users will think it is not worth the effort.

Dropbox’s invite page

But why stop there? Wouldn’t it be great to make inviting addictive? Dropbox does this by sending a nice little email innocuously titled “Dropbox referral status”. What is great about this email is it serves a couple purposes. One, it acts as a re-engagement hook by telling the user how much space they have and reminding them to use Dropbox. Second, and more importantly, it prompts the user to go invite more people. What makes this invite prompt so effective is that it is almost addictive. You just experienced this high moment from earning free space, but you still want more.

Invite completion email

Dropbox’s biggest growth hack though, is giving users an invite incentive that appears to be very juicy, but in reality costs Dropbox practically nothing! This is because almost no one actually ends up using that additional free space. Your storage limit is just a number in a database and doesn’t start costing Dropbox anything until you actually start using it up. Most users aren’t anywhere near 90-100% usage. I’d wager that very few Dropbox users are actually digging into the extra space they got from invites (except maybe those of you who ran a Google adwords campaign).

What they did best:

Enticing “Get free space” invite prompt in top toolbar catches user’s attention

Re-engagement emails prompting users to invite more friends once one joins

Ingenious incentive that has broad appeal, but costs very little

What could be improved: