Hey

When did the word "Hey" replace the word "Hi"? I've been doing it for years. Has the greeting been upgraded or downgraded? When you see someone, is the use of the word "Hey" meant as a more familiar style of greeting or a "passing through the moment and have to say something" type of statement? When was the last time you actually said "Hi"? Is it a lazy way to greet someone or is it the way to show the person you are greeting that they are in your familiar circle and as such formalities are not necessary?

Perhaps it's both? Either way, what the hey?