First. I don't want Facebook to be forced by the government to fact check its political ads, but I still think it's the right thing to do. Allowing the free dissemination of lies isn't a virtue. They may not have a legal obligation to do something, but I think there is a moral obligation.

Also the USPS is a government body and for the most part mail is confidential. Facebook is a private company and the ads on its platform are public facing.