= how many new users on average does one user of your product 'infect' ...A coefficient greater than 1 indicates exponential growth 
Graphic rendition of formula via http://cdn.theatlantic.com/static/mt/assets/science/viral-formula.png
'The virality coefficient is super important, but there's one other critical number that you should pay attention to--the cycle time. The cycle time is the average time it takes from the moment that one of your users performs a viral action to the moment that a new user signs up because of this very action. It makes a huge difference if your cycle time is one day or 60 days.
David Skok of Matrix Ventures gave a presentation about that recently, and actually devised a formula to calculate the amount of users you will get after a period of time based on the Virality Coefficient (K) and the Cycle Time (ct)." (http://techcrunch.com/2011/12/26/eight-ways-go-viral/)
"The problem with Shmilovic's analysis is that it assumes virality is a structural property that can be optimized or reduced to a consistent formula. His recommendations, designed for marketers, are based on creating systems that maximize the space for sharing, differentiated with little marketing buzzwords like "communication virality" ("the product is used to communicate with other people, some of which might be potential users") or "embeddable virality" ("new people who are exposed to the content embed it on their own website, promoting it even further").
The emphasis on structural factors isn't inherently a bad thing: advancements in technology (particularly in communications) have radically transformed the speed and scope of viral products. The Economist's recent exploration of how Martin Luther's Ninety-Five Theses on the Power and Efficacy of Indulgences went viral across the continent through contemporary media -- namely the printing press and multiple translations into the various dialects that permeated 16th-century Europe -- is a perfect (and fascinating) example. In the social space, the prevalence and placement of tools like the Facebook "like" button can certainly be the determining factors of whether a compelling article reaches that tipping point in Shmilovic's Virality Coefficient. The Huffington Post is the ideal model here: the site amplifies its power as a clearinghouse for all things Internet-famous by deeply integrating every conceivable social network and sharing tool into its article pages. When it comes to the promulgation of ideas, infrastructure matters.
But festooning a page with strings of shiny share buttons (Digg! Mixx! Bookmerken! Dipdive!) is a wholly incomplete approach to the spread of information; it assumes that all social behavior and all social networks or online communities are essentially the same. But the human mind isn't a uniform filter, and sharing behavior differs across ubiquitous platforms like Google, Twitter and Facebook. "Nobody can see what you search on Google, so popular search trends tend to reflect the more reptilian brain in people," explained Jonah Peretti, founder of viral hub Buzzfeed, in 2010. ""Celebrity gossip, sex, hair transplants ... nobody tweets about this stuff." A brief glance at the most-shared stories of 2011 on Facebook, Twitter, and Linkedin highlight their differences in focus.
Obviously, the culture of each online ecosystem is shaped by its particular structure, but these have more to do with the how and where of sharing; in reality, it is the why that shapes how ideas take hold. Geert Hofstede, the influential Dutch social psychologist and anthropologist and pioneer in the field of cross-cultural studies, has a succinct take on the role of technology in shaping the spread of ideas and information in his classic work Culture's Consequences. "Electronic communication does not eliminates cultural differences, just as faster and easier travel has not reduced cultural rifts," wrote Hofstede. "The software of the machines may be globalized, but the software of the minds that use the terminals is not.
eople don't engage the unique structure of social networks as blank slates; they enter into each ecosystem with a particular set of values, values that shape the nature of a community and, in turn, the type of ideas and products that take hold. As Alexis Madrigal noted, different networks fill the various social niches in our lives. This is a valuable lesson not just for marketers and media companies, but any person or organization looking to spread a set of ideas or concepts across the vastness of the Web." (http://www.theatlantic.com/technology/archive/2012/01/the-secret-to-going-viral-its-all-about-culture/250641/)