In watching people sample Twitter, Ive come to one conclusion: those who end up finding it a useful service tend to be people who build up a network of a few dozen to a few hundred people. If you only have a few Twitter friends, your chances of getting help when you need a question answered, or having something interesting pop up when you check in, are low enough that the service seems worthless.
Two recent web applications are designed to jumpstart the process by helping you find new potential friends quickly. Twubble looks at your existing friends and recommends other people who they follow. Twits Like Me looks at the text of your tweets to find other folks talking about the same things. Of the two, I found Twits Like Me to be more useful, but theyre both worth a good look if you feel the need for more activity in your Twitterstream.