Maybe it's time to consider Facebook, Twitter, et al, as common carriers, which, under the new net neutrality law, would disallow them from censoring content. https://en.wikipedia.org/wiki/Common_carrier#Telecommunicati...
Whoa buddy, that's some pretty heavy socialism. Anyone can make a forum. Mandating moderation rules creates a restrictive barrier for entry and actually harms free speech that falls out of acceptable range
There's plenty of laws that only apply on certain companies, after n employees or x revenue. Should be pretty feasible to devise a law that applies to sites with more than n active users such as Facebook and twitter and the likes and not to your neighborhood forum.