2014: 43.7% (first two months)
With this pace we will have more than 50% of our visitors on mobile devices in a very short time. Interesting enough, what really changed is that there is very little differences on the time spent on the site, regardless if it's a mobile or PC visitor. We have an average of slightly more than 8 minutes per visitor. In 2011 time spent on the site was significant lower if the visitor came from a mobile device.
At least for our site, this points to the importance of building the sites with mobiles in mind