So, who gets to decide which news is "real" and which isn't?
I see your point, but in reality, the slippery slope is a few miles away from the Pope endorsing Donald Trump.
If people really believe that, then that is a problem that can't be fixed by having someone censor that particular blurb. Trying to do so is simply focusing on the wrong problem, much like trying to eliminate the electoral college and go to popular vote.
In any case, I genuinely think that Facebook and similar platforms are harbingers of doom for a society that wants to maintain democratic roots. Facebook is the "new TV" in that if you want to maintain a rational outlook, and an even disposition, you shouldn't get your news or opinion from it - or probably use it at all. I cannot help but wonder if the complete loss of rationality by the self-identifying left, and right, isn't magnified by Facebook and its ilk. Humans aren't cognitively wired to be well-adjusted when immersed in that type of social environment, nor have we any type of socialized immunity to groupthink or epistemic isolationism.
There are of course, individual exceptions. By and large, my fears regarding "social media" have proven true over the years. But that's another topic.