In this post from the blog "The Innovation Diaries", the author tries to categorize people's tweets into a hierarchy resembling Abraham Maslow's Hierarchy of Needs for human beings. Each category is well explained and then illustrated by a few examples.
Denise Pires, a student from University of Amsterdam, wrote in the blog "Masters of Media" about why we, as human beings, tend to use the well-known microblogging service.