From Pew Research Center: How the public sees, shares and shapes news through social media.

In terms of social news sources a few key facts of note:

• Roughly two-thirds (64%) of U.S. adults use Facebook, and half of those users get news there — amounting to 30% of the general population.
• Surprisingly, YouTube is the next largest social news source — about half of Americans use the site, and a fifth of them get news there, which translates to 10% of the adult population.
• Although only 3% of the U.S. population use reddit, for those that do, getting news there is a major draw–62% have gotten news from the site

In terms of social news behaviors:

• Half of those on social media share news stories, images and video.
• 15% percent posted photos of news events themselves.
• 14% percent posted videos of news events themselves.

Pew Research Center: How social media is reshaping the news.