I read recently that the word Hello came into usage with the telephone. That is, it originated as a telephone greeting. That seemed reasonable to me, as it is the most common greeting one hears on the phone. Today, while greeting a friend in instant messaging, I said "Hey", and then it struck me that Hey is the most common greeting for chat. I wondered if other mediums had their own general greetings, then thought about letters: Dear. Emails don't really have one single main greeting - I've found those who grew up writing letters often say "Dear", which sounds absurdly formal to the rest of us.