There are more than 2 Billion people on the Internet and the reason why viral marketing is taking off in a big way is because most people are connected all the time, either via email, websites and social bookmarking sites.
Reason 1
Viral marketing can be in the formed of emails. Have you ever received an email telling you about how sick a child was and needed some donation to fund for their operation. Or how about the classic scam of which Bill Gates from Microsoft was paying thousands just for forwarding emails to their friends in a tracking exercise.
Reason 2
Have you wondered how Facebook got so big? Viral marketing was being used to it's core. When anybody joined FaceBook, they'll be asked to import their contacts. After keying in their hotmail/gmail accounts and with a click of a button, hundreds if not thousands of invites would be sent from you. Barack Obama was using Facebook to rally millions to donate hundreds of million of dollars.
Reason 3
Have you seen a video being watched by 70 Million people? That is the power of Viral Marketing. Apparently, people find the content so mesmerizing that they feel the need to spread the words to their friends. Google 'Achmed the Dead Terrorist' and you'll see the power of Viral Marketing to its best.