On one hand, CPG marketers have always been wary of social media because they love control and predictability. They are used to drawing on decades of market data to tell them that if they put X amount of dollars into a traditional ad buy on television, radio and/or print, they have a pretty good chance of getting Y level of return. Social media, as a relatively young and evolving channel, doesn't have that kind of predictability. That tradeoff tends to weigh heavily when it comes to determining marketing spend for a brand with sales expectations upwards of $1 billion per year.
On the other hand, social media has also always held an allure for CPG marketers. It appeals to an aspirational desire for consumers to take ownership of the brand. Diet Coke and Zappos are prime examples of instances where consumers have created their own messages on behalf of a brand because and in doing so create a sense of ownership for themselves. Not only does this type of messaging and behavior tend to benefit a brands' reputation; once a consumer feels they "own" a brand, it is very hard for them to be convinced to switch to a competitor.