First, Google don’t do this. It could provide now some tools to fact checked some news related to this election, but really after that it would return to the lawless way it was before. So, don’t oversell Google as the good guy as this is not a thing that would be with us for so long.
Second, who would pay for it? Not all contend can be fact checked by an algorithm, so human intervention would be required, something that not only cost money, it’s something Facebook wouldn’t do period. With the millions of post it would required an army of fact checkers, that besides to be costly, can bring her, his, hex bias to the job. They’re humans after all.
Third, you really thing people would use Facebook knowing that someone it’s reading everything you are posting? We fear big brother, other if it’s the government or a private entity.