In retrospect, the killer feature was the "Sign Off" button.AOL Instant Messenger was synchronous. Unlike texting, seeing a "buddy" online generally meant they were present and open to chat. After a while, you left (as though from a real place), and the sound of a door slamming shut confirmed your exit. You were no longer there, and everyone knew it.Life was good. Then we made chat asynchronous.You may think iMessage is the spiritual evolution of AIM, but they are not the same species. In an asy