While mobilizing fans can bring huge benefits to a brand, companies aiming to promote their brand by generating viral exposure to their content are gambling on a risky strategy. Some brands do have a lucky streak with content that was designed to go viral and succeeds in doing so. Their chances of sustained growth, however, are small. No professional brand manager should build a marketing strategy based on this approach.
 
The facts are clear. Very few marketing initiatives go viral. An analysis by market research company Kantar Millward Brown (2011) found that 96% of US ads achieve a trivial number of organic views on YouTube.

Only about 1 in 100 ads achieve significant coverage from f'ree’ views, and these ads tend to have already benefitted from 'seeding’. Seeding is the investment made by an advertiser to kick-start exposures to an ad so that it appears in 'Trending Videos’ lists, encouraging further views.
 
The experiences of UK chocolate manufacturer Cadbury from 2007 to 2009 illustrate why brands cannot rely on viral exposure for long-term success. In 2007, Cadbury aired its famous 'Gorilla’ ad. It featured a gorilla playing the opening drum sequence from Phil Collins’ hit ’In the Air Tonight’. The ad was bizarre, mesmerizing and highly sharable. It was also profitable - if you factor in distribution gains and price effects. Cadbury tried several times to replicate this success but without much luck (e.g., 'Airport Trucks’ in 2008 and 'Eyebrow Dance’ in 2009).

Check out this LinkedIn thread for a discussion about the topic.