Six Principles of Viral Marketing vs Hotmail.com - 2

2) Provides for effortless transfer to others
Attach a simple tag at the bottom of every free message sent out: "Get your private, free email at http://www.hotmail.com". A short and clear message can be easily to transmit without degradation.

3) Scales easily from small to very large
Message spread like wildfire the transmission method must be rapidly scalable from small to very large. Hotmail.com used this element to attract netizen to use this free e-mail service, then stand back while people e-mail to their own network of friends and associates.

4) Exploits common motivations and behaviors
Design a marketing strategy that builds on common motivations and behaviors for its transmission, hotmail.com targeted "Who see the message" and encourage netizen to forward their e-mail to others for attracting use.

Reference: http://www.wilsonweb.com/wmt5/viral-principles-clean.htm