Mass media like TV, radio, and newspapers can help you reach the masses (if you have the budget, that is), but social media can help you reach targeted influencers. The American Red Cross knows that recommendations from friends and relatives (including those posted on social networks) have more influence on donations than ads or fundraising campaigns. By mixing network analysis (who is influencing whom) with sentiment analysis (insight into what they're saying), companies can reach the most important influencers. You can use services such as Klout or built-in features available with some sentiment analysis platforms to figure out who the social network influencers are for a particular topic or industry.
In some cases social channels can approach TV reach if an influencer happens to be a celebrity. If actor Ashton Kutcher tweeted about giving blood, for instance, he'd be instantly telling his more than 11 million followers about his good (or bad) experience.