Facebook will use "fact checkers" to censor posts. But, who will check "fact checkers"?

Facebook now flags and down-ranks fake news with help from outside fact checkers like Snopes, FactCheck.org, Politifact, ABC News, and AP. Some of them we know that only fact check the stories that make Dems uncomfortable.

So Mark Zuckerberg wants to be “the arbiter of truth". But, who will check him and his "fact checkers"?