I recently joined the YALITCHAT online community. This is an excellent community of young adult book writers, editors, agents, publicists, marketers, packagers, librarians, sellers and readers. As of this morning there are over 1000 members. I've joined four groups inside this community so far.
Does your query need work? Are you looking for several more opinions on it? Then check out the Query Kick-Around group.
You can get the latest news, scoop and insider info on agents who represent YA in the Agent Insider group.
At the Agent Inbox, you can submit to agents who have chosen to accept #yalitchat member queries along with your first 250 words. The moderator vets the submissions based on personal agent preferences and active relationships with each agent. You may receive feedback from agents.
I also joined the Queries that Work group to read about successful queries, including the actual results - the agent or deal, the release date, partial/full requests, etc.
There are so many other groups and activities you can participate in, such as blogging, discussion forums, events, contests, chat and more. I'm darn near ecstatic at finding it. Now I just need to more regularly participate in the weekly #YALITCHAT discussion on twitter.