Are e-mail messages, forum postings and social media updates protected by copyright? Shouldn't the author assume that they will be shared?

These are protected by copyright just as any other writing that is established in a fixed format, unless the author states otherwise. Some listservs make it clear that, by posting messages, the author is placing them in the public domain, but not all do so. If in doubt, it is best to ask for permission or to apply the fair use criteria. You may however paraphrase the messages.