They are talking about FAKE stories, not slanted stories
Yes, what Fox News says about something may not be the same as what MSNBC says about it. But that's a world of difference from making something up, like the the stories that had the Pope endorsing Trump or an FBI agent investigating Hillary's emails murdered under suspicious circumstances. There's no slant there, these are just complete fabrications.
As for the purveyors of these fake stories marking real news fake to try to muddy the water, one would hope not just anyone will be able to flag a story as fake. Otherwise a negative but true story about Trump would be flagged as fake by his followers, a negative but true story about Hillary would also be (or would have been, no one cares now that she lost) flagged as fake by hers.
I would guess maybe people can flag a story and if enough do someone at Facebook will take a look at it. If it references some newspaper that doesn't exist, who is a fake page that claims to be Fox News or CNN but actually isn't, or quotes a NYT article that doesn't exist, it will be easy. If it is some fringe site that makes stuff up, it will be a little more difficult, but presumably if a certain site has a few stories marked fake they could use that knowledge to automatically flag other questionable stories from there - an incentive for such sites to clean up their act and not repost fake shit.
It won't be easy, and it won't be without controversy, but there are too many brain dead people who see a story against the politician they hate and automatically like and share it not knowing or even caring if it is false. I saw one guy I know is pretty smart (owns his own business worth millions) sharing obviously bullshit stories in the last few months. I assume he knew what he was doing and was just trying to muddy the waters, but maybe he was so brainwashed by Hillary hatred that he really believed the NYPD was investigating her for being part of a child porn ring and other batshit crazy stuff.