I wrote about this in an earlier post, but I think it's worth a reminder. Viral Marketing is not:

Telling your friends to Tweet about your new site.

Posting about your upcoming concert on Facebook.

These are simply modern ways of doing old fashioned push marketing.

Viral Marketing is when:

one person tells two people about your product (3)
each of the two tells two more people (7)
each of the four new people tell two more people (15)
each of the new 8 people tell two more people (31)
each of the new 16 people tell two more people (63)
each of the new 32 people tell two more people (127)
each of the new 64 people tell two more people (255)

You get the idea.

Viral marketing is when word about your product spreads, naturally, like a virus; and frankly, without much effort from you. Other than making the product worth talking about.