A person that follows you around and updates your Twitter for you so you do not have to.
Person 1: Who is that?
Person 2: Oh, that is my Tweetist. He is making sure that my friends know what I am doing all the time.
An obsessive-compulsive twitterer that spends an exorbitant amount of time, and effort, either tweeting for themselves, or on the behalf of others, while believing it to be normal behavior.