Why do people pretend this is some kind of deep philosophical conundrum? In this case Twitter, who hires the fact checkers, and the audience, who comments on what the fact checkers do, or is free to leave the service. Your comment and the article you are commenting on and this very discussion is 'checking the fact-checker'.

Same way accountability in any system with many actors works, in reciprocal fashion.

No one is pretending that it presents a philosophical conundrum -- it is exactly that.

The vast majority of people would appear to miss this conundrum entirely and thereby never consider the fallibility and corruptibility of fact-checkers and/or experts.

If you do not agree with this viewpoint fine, but to try to attribute deceit ("pretending" in your words) to anybody who thinks differently about it from you seems really odd and highly presumptuous to me.

In these cases where there's an obvious customer of the fact checking service, it is really obvious who is fact checking the fact checkers.

The condundrum isn't who, it's whether they're doing a good job.

> who fact-checks the fact-checkers?

Fact checkers are just a name like antifa, to make it sound like if you are against them, you are anti-fact and therefore a crazy person. Smart play with naming.

Fact checkers are just "narrative control".

We don’t ask questions like that around here, it induces vaccine hesitancy and sounds like misinformation. And we like to not ask questions that make us look anti mainstream.