Viral marketing is a concept which allows word of mouth advertising to spread your message far and wide without ongoing maintenance and promotion from the originator.
Hotmail is the most famous example I can think of to show you the power of viral marketing.
Everyone who used their free email service would advertise the service at the bottom of the each email they sent out.
The types of advertising messages which grow the fastest are services which advertise themselves when being usedsuch as the Hotmail example.
Another good example is a service such as http://Plaxo.com
This is an online address book. When a user signs up, the service scans the users address book in the users email client and emails all those users an invitation to join Plaxo. This is done with the members permission, of course.
Other services which use this technique of viral marketing are social networking sites such as http://Myspace.com
When users sign up for a free account, the first thing they see is a form to invite their friends to the site.
I’ve deducted a few tips from following high profile and low profile viral marketing campaigns of many...