ReadWriteTweet: a Twitter directory of YA authors, characters, and reviewers. (Literary agents too.) You can add or update a tweeter or follow me @readwritetweet.
Have fun.

Read Write Tweet is a growing directory of literary agents and YA authors on Twitter, as well as YA reviewers, news, and characters. Authors and agents are tagged with the genres they write in or represent, for easy browsing.

Current listings in each category
YA Authors: 289
YA Characters: 14
YA Reviewers: 38
YA News: 21
Agents: 94
Info: 13

The @readwritetweet Twitter account follows everyone in the directory, and retweets news, contests, advice, and other things of interest. For recent activity and daily summaries, hit the tweet archive.

To add to or update the directory, just fill out the form.

Last five additions:
Agents: Brown, Robert
Agents: Lee, Kate
Agents: Shults, Lauren
Agents: Wolfson Literary Agency
Agents: Red Sofa Literary