They are arbiters of truth whether they choose that role or not. The question is only who (or what) chooses, andd how well they do that job. The obligation cannot be shirked.
The problem is that Facebook was presenting truth and lies in identical ways. People should be able (and willing) to dig in and find out what is true for themselves, but that takes time and effort.
If Facebook itself does not, then those with the power to game Facebook itself will do so, because there is a gain to be had by disseminating disinformation, propaganda, distraction, fomenting distrust, etc., etc., etc.
What you actually do want, whether you realise it or not, are well-formed, well-behaved, epistemic systems. Note that the original false dichotomy I referenced was between no regulation of this and unaccountable regulation.
The fact that that is in fact not the entire universe of possibilities seems to be being studiously ignored.
Whatever metric they choose is almost certainly much more likely to be gamed by powerful people. One doesn't need to look much further than PropOrNot, or the Google changes to address supposed fake news, or the immediate reactions to the Russia indictment, to know that any drive to "eliminate fake news" will immediately be turned against independent media and causes like BLM, opposition to the Dakota Access Pipeline, any political figure not popular with Beltway types (like Sanders or Stein), etc.
I do enjoy being told what I actually want, though, so keep using that not-at-all-patronizing rhetorical maneuver.