...a reputable auditor verifies the source of data and process, reviewing evidence directly from the systems involved. This is standard for anything, it's why we have CPAs to ensure companies aren't cheating investors. If it's really true, then a third-party should be able to verify the results.
But it doesn't matter because Facebook obviously isn't interested in stopping any clickbait like fake news since they depend on the revenue it provides. If they were, they would simply create a whitelist of all reputable news sources which all share the same traits regardless of bias accusations: they have qualified editors and journalists (no, random bloggers who copy/paste/scrape news from the AP wire are not editors or journalists), do fact-checking and verify sources per journalism best practices, publish retractions/corrections as needed, and mark opinion pieces clearly as such. They can come up with whatever criteria they want to make the bar high enough to filter out the clickbait crap. It's really not that hard.
On a side note, I don't know why anyone believes anything that has more likes or followers (aka whenever something "goes viral") has more value since it's highly likely it's all being manipulated by paid spambots.