And see, I'm not the luddite type at all. I barely use Facebook (I check it maybe once a week), but I have an account and I actually see some real use value in it. I appreciate being connected to my friends, especially those I fell out of touch with after high school/college, and now that I live in South Korea, it's an easy way to see what people are up to, and to let people in on what I'm up to, as well as find groups of people doing interesting stuff in my city. I'm not crazy about Facebook by any means, but it is nice to have available.
Twitter, on the other hand, would have absolutely no use value for me. I think Tumblr finds a happy medium between Twitter--which is a buzzkill if you ever have a thought that runs over 140 characters--and most major blogging services--which give you clunky layouts and options, and then make them a pain the ass to use/customize. With Tumblr, I actually feel like posting because using the service isn't a pain the ass. I can make it look how I want. I can post a picture without having to make sure the pixel width is perfect. And the community features are just present enough to be pleasing without forcing you to invite interaction on your own blog (commenting isn't packaged with the service, though you can sign up for and add commenting code to your blog if you please).