The term was made up to describe completely and deliberately made-up news stories created for lulz or clicks, but immediately expanded to also include news that isn't deliberately falsified but might just be biased, distorted, true yet deceptively phrased, or non-deliberately inaccurate -- and from there used as a general catch-all term for criticising any news source you don't like, whether that's Breitbart or CNN.
There is very little objective news left, and very few unbiased umpires remaining to judge it. I don't think any of us should trust facebook, of all places, to set itself up as the arbiter of what is "fake" and should be censored, as the company's own political views are well known and I don't believe they are likely to be capable of applying consistent standards to both sides.