There’s nothing new in publishing anonymously, just ask George Elliot. What’s new is the notion that publishers have no liability. Social media companies do not claim to speak for themselves. They have no reporters, no sources to protect. They’re one giant “letters to the editor” section. They should know for whom they speak.
Whether or not a writer commits libel is for the courts to decide. Neither the writer nor publisher has the right to avoid responsibility by camouflage.