I came across this great and still relevant infographic sponsored by compendium.com, marketingtechblog.com and exacttarget.com.
It shows that by 2016 80% of the US population will use smartphones, and 50% will own a tablet. Although more users are checking emails with their phones, the open and click-through rates are still higher on the desktop. Perhaps it is time subject lines were optimized to take advantage of the growth of mobile.
Email on mobile also seems more useful for lead or conversion generation than Social and Search. Like I mentioned above, excellent statistical information in this infographic.