A person who is addicted to twitter. They tend to tweet at least 20 times per day, follow at least 200 people, and attain at least 150 followers. Since twitterholics tend to do very little other than use twitter, Twitters intended purpose of quick messages about what you are doing is not enough to satisfy their need to tweet, therefore they carry out conversations, share links, and play games (like Colorwars). Twitterholics are competitive about their stats, trying to post more than anybody else, or have more friends/followers than anybody else (progress can be tracked on twitterholic.com).
Person 1: Hey, wanna go do something?
Person 2: Can't, I gotta tweet a bunch, one of my friends just got to his 10,000th update and I'm still at 9,500!
Person 1: Dude, you're such a twitterholic