As you can see from the list of links to the side, I'm part of several social sites, including Twitter. Now, I am not really active on all of them - I'd have no life if I did that! But a couple really appeal to me, plus I've gotten results as far as book sales, contacts and networking.
DeviantArt is my favorite. I also manage an online writer's group there, which is ideal for authors on blog tours. (We've got over 400 members...) I connected quickly with this artistic community and have readers and fans all over the globe because of DA.
I'm also on Facebook, which is a vast improvement over MySpace. I do have a MySpace, but only visit 1-2 times a week. A tip from author Ellen Hopkins made that experience better the second time around, as the first time I had a "perverted weirdo" from MySpace show up at a local book signing. (Now my photo is NOT on my main page - it cut down on the weird0s..)
And I confess, I like MyCatSpace, too! What can I say?
So outside of blogging, what communities do you enjoy?