Hey, I've used twitter but didn't try facebook to promote my site. I always think that it will be nice if I start promoting my site with facebook because lots of people are using facebook now. If you want to ask my experience, twitter can drive some quick traffic to your blog/site but you really need to subscribe to relevant group as you don't want to get poor quality traffic.
I prefer FaceBook over Twitter as i dont see much interest in Twitter it has become too over crowded with businesses trying to advertise which really makes Twitter lose the idea of Social Networking.
As for me I do like facebook because of farmville and other nice application they have got in their site right now. At the same time my friends are growing because of this games
I do prefer Facebook because: 1. I have a lot of friends there. 2. Their wall post are the greatest idea ever. 3. My traffic comes from Facebook group and fans page.