I'm not certain what you mean by a "longstanding judicial principle." Are you conflating the constitutional provision protecting freedom of speech, which prevents the federal and state governments from restricting free expression, and the authority of private publishers to control the content of their media?
I think private *social media* publishers have a problem either way. The could probably assert the right to control expression on their private system (as newspapers and television networks do) but if they did that, then they'd open themselves to numerous claims that they didn't, in some cases, do the right thing. If X libels Z, and the libel doesn't fall into certain protected categories, the publisher is liable for damages (as they are with newspapers and regulated TV networks.) In other words, if they took some responsibility, they'd take all responsibility -- and that would be tough on a social media network that may have millions of comments in a single day.
However, social media benefit from a "carve-out" which treats them not as ordinary media, but basically as a kind of telephone system which simply transmits what is said, and is not responsible for monitoring it. Therefore, if X libels Z, Z can sue X, who might not have a dime to his name, but can't sue the deep-pockets publisher. The reason for this is that the social media argue that with tens of millions of messages sent every day, they literally *can't* monitor it all. But when outrageous things are said, the question arises, how can this kind of obviously libelous, dishonest commentary be allowed, considering the damage it is doing?
Frankly, there have been some comments made on this Coffee Corner series of threads that have veered closed to libel; ordinarily, X could sue Z, but couldn't sue LULA. I don't know what effect the presence of a moderator would make. Does his presence mean that LULA has assumed responsibility for what is said on these forums?