Sociologically, twitter is very different from other communication forms. AIM and Skype are designed for communicating thoughts and ideas. Twitter, however, is designed for telecocooning. Telecocooning is traditionally defined as sending little "presence pokes" to people. For example, just saying hi or sending someone a picture of a heart or a smiley face. The point of these messages isn't the ideas they contain, but rather the message itself. That is, just getting any message from someone shows that the other person is thinking about them and cares about them.

Traditionally people telecocoon with their two or three closest friends. However, Twitter lowers the telecocooning barrier so you can telecocoon with many more friends for the same effort. People tend to just send messages like "ahh homework sucks" or "heh stuck at the library." You aren't really communicating anything intelligent, but you are building a feeling of group solidarity by letting people around you know what's up. It also helps people to find where their friends are if they want to meet up.