“Email, on the other hand, has a certain unfair advantage in that shoppers getting the emails have already given up their addresses to a site, suggesting they already have some prior relationship with that retailer. Still, despite the avalanche of spam we all get, it’s easy to see how the staying power and greater potential for personalization of a medium without a 140-character limit gives email distinct advantages.”

http://www.wired.com/business/2013/07/email-crushing-twitter-facebook/?