To use your example, whether I put up a website or a billboard with defamatory information is irrelevant. We have laws that protect us, which declare that defamatory speech is not protected. Why does it matter that the speech is online or not when considering the essence of the law? As you stated, if you put up a billboard they can find you and sue you for defamation. Why shouldn't the same be possible online? If I get an injunction against a billboard with defamatory speech the owner of the billboard is required to take it down. Why shouldn't an online provider be equally obligated?