Facebook is already arbitrating the truth. If you share a link or post an ad, it can be automatically annotated by an "independent third party fact checker", which they have a few of. But if it's an ad that is specifically from a political candidate, there is no fact checking done. Of course, these ads are mixed into your news feed with just a small "Sponsored" note.

If Facebook had not started its fact checking system nobody would be asking it to fact check political ads now.