Kathy Sierra has a great post on some of the psychology that plays into Twitter. I had started to go down a very similar line with my earlier post about the Twitter reinforcer, but she does a much better job (plus she even has a graph).
I think the psychology around Twitter is fascinating. Since it does combine elements of blogging, IM, texting, and even email, it’s a hot bed of possible research. Psychologist and sociologist have been studying human interactions for ages, but online human interactions (social networking) are different and will require their own areas of expertise. As humans start to maintain relationships offline, mixed offline and online, and completely online, we will all need to realize that different expectations need to adhered to given the context of the relationship, the medium of the communication, and the public vs. private nature of each message.