Viral marketing:

The buzz words "Viral marketing and Viral advertising" refers to marketing techniques that use pre-existing social networks to produce increases in brand awareness or to achieve other marketing objectives (i.e 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 promotions may take the form of video clips, interactive flash games, ebooks, brandable software, images, or even text messages. The goal of marketers interested in creating successful viral marketing programs is to identify individuals with high Social Networking Potential (SNP) and create viral messages that appeal to this segment of the population and have a high probability of being taken by another competitor.
Notable examples:

• In 2001, Business Week described web-based campaigns for Hotmail (1996) and The Blair Witch Project (1999) as striking examples of viral marketing.
• Launched in 2002, BMW films was among the earliest viral marketing campaigns. It attracted nearly 55 million viewers.
• Cadbury's Dairy Milk 2007 advertising campaign was heavily popularised on Youtube and Facebook.
• Burger King's The Subservient Chicken campaign, running from 2004 until 2007, was cited in Wired magazine as a striking example of viral or word-of-mouth marketing.