Four Viral Loop Drivers [updated]

I was noodling about different categories of viral loops and started making a list…

Viral Loops typically center on:

1. Contact Lists — e.g., inviting new people thru the process of finding/adding new contacts to a contact list
2. Conversations — e.g., inviting new people by adding them to chats, events, groups, etc.
3. Media — e.g., inviting new people by sharing media/content with them
4. Incentives — e.g., inviting new people to unlock functional or financial incentives

Any others that you can think of?