Although not a new concept, creating "viral loops", or figuring out how to boost your "viral coefficient", is still the fastest way to grow.

A viral loop is identified when a user takes an action that gets, on average, more than 1 person to take the same action. In theory it represents infinite growth, in practice it means very, very fast growth.

Consider the following viral loop example:

1. User A signs up for your product. During the last step of signup they are presented with an incentive to "invite their friends".

2. On average, that person invites 10 people to try your application.

3. For every 10 people that are invited by a friend, 1.2 (or 12 out of 100) people sign up and go through the same process

How viral your application is will be based on how many viral loops you can create, and how long it takes to complete the loop. The faster it is, the faster you'll grow.

Brainstorm all possible viral loops you can introduce in your application or service. Here are some examples:

1. After signup, invite a friend

2. Offer an incentive for successful "referrals"

3. After the user completes a key task, ask them to share or invite friends

4. Create a "point system" (sometimes called "gamification") that rewards users for recruiting new users

5. After a period of time, you ask each user to rate your product and share it on social media sites

Use a whiteboard to capture all of your viral loops over time including inputs (when they enter a loop) and outputs (where they will send users):

Look for ways to optimize your viral loops by providing additional incentives to "share" or refer, and making your core product more viral.

Here is a fantastic article on Viral Loops and the viral coefficient by David Skok.