Rob told you last week, as did many others, about the reports that Facebook employees regularly spike news stories that are favorable toward conservatives or have a conservative point of view.
No one who spends time on Facebook regularly should have been surprised by this. It's very obvious that their "trending stories" section is put together by people who lean to the left. Everyone here believes that, while Facebook should not pretend to be unbiased when they clearly are not, it is Facebook's prerogative to run its business any way it sees fit.