Andrew Chen sums it up perfectly,

Viral marketing is not a marketing strategy
Many times, viral marketing is seen as a “marketing strategy” that is interchangeable with other methods of acquiring users. That is, you go through three steps:

1. Develop your product
2. Think through a plan on how to make people use it
3. Declare viral marketing is one of N approaches (along with SEO, SEM, PR, etc.)

Forget about adding “viral” to your marketing to-do list after your
product is already on the market. You need to bake it into your
business model from the very beginning. “Viral isn’t something you can
just make happen,” says Botha. “It has to be inherent in your product.”