Definitely Tumblr. I love the art communities there a lot more, and just love the fandoms and fans there. It depends of course on what you like, but overall I am loving Tumblr, and have found fantastic artists there, and am inspired there everyday.
I used Facebook a lot, but it's not been a favorite for a long time. It's gotten so useless lately; Nothing but reposts, images with text on them, and very little in the way of actual status updates. The eCards with sassy, reused jokes on them have gotten a bit old. Every so often I'll see one that amuses me, but not as often any more.
Twitter has grabbed me lately. I like feeling that people I am a fan are just a click away. I always get a little giddy when I get a response from a voice actor or artist that I'm fond of.
Tumblr has never been that big to me. I use it occasionally, but not often.
Really like the format of Google+, but I don't check it often. Steve Wozniak has me in one of his circles!
I've joked about going back to MySpace just to enjoy the peace and quiet.
Tumblr because the format makes it so much easier to see a lot of content in a short amount of time. I actually like submitting my art on tumblr because when other people reblog it, you're reaching a whole new audience that follows those other people. With deviantArt, only your watchers see it, unless you reach near the front page, which never happened for me before.
dA is the only site I'm on, constantly, every day. FB is just kind of...there. Check it a few times a week and that's it. Tumblr I just don't see the appeal, at least as an art site. It seems to be less functional for art viewers (I can't speak to the submission part of it) than dA in every way.