Take for example Facebook's top ten for the last ten days, per the New York Times' Kevin Roose's tracker. These are self reported by Facebook's CrowdTangle:
Today, 10/16, most of the news is explicitly partisan and right leaning, one post by Dan Rather: https://twitter.com/FacebooksTop10/status/131751290168337612...
10/15: similar mix, https://twitter.com/FacebooksTop10/status/131711759703326720...
10/14: https://twitter.com/FacebooksTop10/status/131674996947526861...
It goes on and on. Scroll through the Facebook Top 10 account and you'll see right leaning, sometimes far right pundits, at greater volume and (if these links follow the typical power distribution) far, far greater attention and traffic than any even moderate or neutral news source, let alone anyone comparably left wing.