For those of you that are looking for a definition read this just below. If you want to see the top 10 viral ads of 2008, skip to the bottom…
Viral marketing and viral advertising refer to marketing techniques that use pre-existing social networks to produce increases in brand awareness or to achieve other marketing objectives (such as product sales) through self-replicating viral processes, analogous to the spread of pathological and computer viruses. It can be word-of-mouth delivered or enhanced by the network effects of the Internet. Viral marketing is a marketing phenomenon that facilitates and encourages people to pass along a marketing message voluntarily. Viral promotions may take the form of video clips, interactive Flash games, advergames, ebooks, brandable software, images, or even text messages. The basic form of viral marketing is not infinitely sustainable.
The term “viral marketing” is also sometimes used pejoratively to refer to stealth marketing campaigns-the use of varied kinds of astroturfing both online and offline  to create the impression of spontaneous word of mouth enthusiasm.
from Wikipedia: http://en.wikipedia.org/wiki/Viral_marketing
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