I can't really pick one or the other. I like Facebook because many of my real life friends use it, and it's a full website. I like Twitter because it can instantly improve your website's page views. If I had to give up one or the other, I'm not sure which one I'd choose!
Facebook...sorry twitter can be such a freaking pain in the arsh when trying to get answers to questions...or maybe I am following the wrong people...at any rate I love facebook more ;)