Slander on Facebook

This screenshot comes from a community Facebook page and, aside from being funny when you read what motivated the potentially defamatory content, it demonstrates issues with moderation.

Elsewhere I've seen a tattooist defamed on a local newspaper's Facebook page at an early hour when the moderator wouldn't have been present to delete the post.

It made me wonder who would be responsible in a defamation case. Would it be the poster or the publisher? If the latter, is Facebook the publisher or is the page moderator at fault for comments?