It may be true that it's hard to draw a clear line between bullshit and almost-bullshit, but it's hard to draw a clear line between porn and almost-porn and they somehow manage to do that okay.
edit because apparently this wasn't clear enough: the way you distinguish science from bullshit is not by evaluating the claims (that's what I am arguing we should not trust Facebook to do). One is a money-making endeavour and the other isn't, and that's the basis on which they are distinguished. Even the worst science is not festooned with ads.
I'll say it again: "Facebook doesn't do a good enough job of evaluating accuracy" is a trap, and it's a trap they desperately want you to fall in to. If we get to the point where we put any value in FB's evaluation of any scientific claim, we're already into dystopian sci-fi territory, no matter how good a job they do.