A more sophisticated answer can be found in this article by Paul Hyman (ACM News, January 12, 2012), which is a report on a technical article (Modeling the adoption of innovations in the presence of geographic and media influences by Jameson L. Toole, Meeyoung Cha, and Marta C. González), but from me you're going to get a very minimal, two-part explanation.
- Lots of people who lived near each other adopted Twitter early on. The service became available in late March of 2006, and by early August of 2009 "nearly 3.5 million people signed up for Twitter, mainly in cities with high concentrations of young, tech-savvy early adopters like San Francisco and Boston."
- Celebrity praise of Twitter - Ashton Kutcher and Oprah Winfrey are mentioned - also boosted the number of users dramatically.
So if you want your product to "go viral," release it first in locations with lots of likely early adopters and induce celebrities to plug it. It would probably help to use Twitter or Facebook.
Ken Pimple, PAIT Project Director