A facebook fanpage thingie is good, IMHO, because people can like it and share it and it spreads... A few artists on here have both a personal page and a fan page and I kinda wonder about the personal page, because you would be inundated with friend requests from dA fans.....
Not only do I have one but I have a Fanpage set up for my comic.
It's definitely a way to tag another group of folks. I know people who live on FB. I also know folks who don't care for it and live on Twitter and Tumblr. The ability to cross-post from one source to the rest is always a plus as I can post to my fanpage and have it hit Twitter or start from Tumblr and have it sent through the whole chain.
Blanket the networks. Leave no fan alive... er, "uninformed"! = P