Create an incentive for a user to invite their friends, and for their friends to accept the invite. Rewarding both sides is crucial — if the incentive is missing for either side, it won’t catch.

In the early days, LinkedIn had a similar thing going. By showing the number of connections front and center on your profile, they incentivized you to start thinking about inviting more users to connect with you.

Apps built for collaboration or communication between coworkers are inherently viral. Some have a single user mode, but additional value is realized from the product with multiple people.

Create the ability for others to embed your product into their website. This will create exposure, and also model to potential users the expected content and behaviors in your product.

social sharing. If your product creates uniquely interesting content, users can be encouraged to share it on their social networks, thus spreading knowledge of your product to their network. The most successful examples in this genre is auto-shared content onto other networks.

Chris Dixon calls this “Come for the tool, stay for the network.” You create a uniquely appealing artifact, and make users want to share it on their networks.

Your product generates artifacts or URLs that are expected to be shared via messaging. Your product provides a rich experience around the information that’s shared.

Folks that are about my age usually think of Hotmail’s signature line at the end of every email, which helped Hotmail grow from 20k users to 1.5M users over the course of 4 months in the early days of the web:

Viral loops contain the user spreading awareness and consideration of your product. But conversion and loyalty all have to happen based on the quality of the product:

virality is something that you have to build into the core mechanics of your product from the start — in most cases you can’t just glue it on later.