Freedom of speech includes the freedom to not say something. It would require a constitutional amendment for the gov't to come in and start demanding things from Facebook.
well, i don't know if thats the case in good ol'merica.
But i don't think there are major obstacles to introduce such law in germany/europe. We already have media-laws and special press-laws (which regulate how stories must be published, for example we have a "journalistic duty of care" which amongst other things mandates that unconfirmed statements must be marked as such. The law follows the codex of the German Press Council).
