There is a bit of a difference here between Facebook and news sites. Facebook has a user agreement. They publicly also say the do not skew information. Now there is evidence they do. If they violate their agreement, seems an ingenious attorney should find cause for action. Consider the grief they have had over privacy rights. Tie the two together.
I just read mime...
I didn't see much in their agreement about what criteria they use to post information...
Of course, they have several different agreements for different services...
It might be in there...I'm simply not that vested to read them all...
I also will add each user has a "news feed preference" on their page, they can tweak to their liking...
I have no doubt Facebook leans left, but my point to you was suing them for bias won't work...