1) The document discusses why analyzing Twitter usage data is important and provides examples of prior studies that have analyzed Twitter usage patterns and categorized tweets.
2) It identifies major categories of tweets including conversational, status updates, information passing, news sharing, and spam. It also describes minor subcategories.
3) The document presents the results of categorizing the author's own Twitter archive between 2007-2009, finding most tweets fell into conversational and status update categories. This showed Twitter is often used for personal exchanges not just broad broadcasting.