For me, it depends on who I'm writing to. Even on facebook, if I'm messaging a friend I'll start the message with "Hi," and a line break. I never use emoticons, sentence fragments, or slang in client emails, it strikes me as very unprofessional. For first contact in professional communication, I usually start with "Dear Mr. X," and a line break. It does get confusing, though, when they reply with an email beginning with "Michael:" I never know if I should respond to that with an email also greeting them by first name, especially if they are much older than me. I think its a grey area, and just comes down to what the client or recipient is expecting.