It's not so much a gray area that needs solved as it is something that needs to be determined on a case-by-case basis. It's something so nuanced that it takes a jury to decide. Many laws are like this.

The litmus test should be this (for libel, as an example): if a reasonable, sensible person can have a greater impression of truth by that statement being on your website versus being published directly on the author's own website, then you should share part of the blame.

That is: in early Facebook, no sensible person would believe that a statement, just by being on Facebook, is more likely to be true. But, in modern Facebook, if a sensible person sees something a bit "spicier" posted on Facebook and widely shared, something that would normally be quickly "fact checked" or outright removed, and it is not fact checked, it could be reasonable to believe that the statement is so true that even Facebook wasn't able to find something fallacious about it. You could easily find yourself in a situation where you believe something is true because it's uncensored on Facebook that you wouldn't otherwise believe.