About 50% of YouTube's views come from mobiles or tablets

"YouTube CEO Susan Wojcicki appeared at the Code/Mobile event on October 27 and shared some stats about the online video site. One of the most interesting numbers concerned mobile devices, which now supply about 50% of YouTube’s views.
This is up 10% from the last time YouTube reported any stats related to mobile viewing. In 2013, the Google-owned video platform claimed roughly 40% of the site’s views came from mobile devices. And a year before that, in 2012, only a quarter of its views were from mobile.
“Mobile is super important. I think it’s important for every business right now,” Wojcicki said to Re/code at the Code/Mobile event."

Source:  TubeFilter, 28th October 2014
Update - Confirmed by Susan Wojkicki of YouTube in July 2015