Twitter’s not a site, it’s a protocol. Twitter-like micromessaging is a relatively new communications model, with unique characteristics that affect how we use it and what’s appropriate.
It’s an RSS feed for people, a way to directing the attention of audiences, and a means of reaching the famous without burdening them with an obligation to respond. In short, Twitter is a human API. It’s being defined in real time in front of our eyes, through an amazing example of Internet Darwinism. More on the email debate « Kellypuffs - Not Just for Breakfast A. Ed Brill on Sametime.