Viral marketing is the main source of getting traffic on website.
Viral marketing depends on a high pass-along rate from person to person. If a large percentage of recipients forward something to a large number of friends, the overall growth snowballs very quickly. If the pass-along numbers get too low, the overall growth quickly fizzles.
At the height of B2C it seemed as if every startup had a viral component to its strategy, or at least claimed to have one. However, relatively few marketing viruses achieve success on a scale similar to Hotmail, widely cited as the first example of viral marketing.