These days i only use Twitter and YouTube although i do have an account with Facebook i have never fathomed out how to use it too its full potential,the main one i use these days is Twitter.
YouTube and video marketing is the way to go. If you manage to become someone on youtube then you are set for life. Then, get a fan page on facebook and a twitter. But YouTube has to be the main one. Trust me, i've been there.
I only use twitter and believe me if you know how to remain on top of twitter by regular quality tweets then you you can get lots of profits from them
As most everyone has said, I think a combination of the three (and more if you can) is the best way to go. I personally use Twitter, but I'm still growing and haven't seen it's true potential. I am on Facebook for friends and family (and FarmVille), but not for marketing, yet. I've never been active on YouTube, but I have seen it's potential first hand, and for the right crowd I think it is a goldmine.
Facebook has driven a lot of traffic for me ... twitter is also good ... As for youtube ... u need to make a attractive video to convience to viewer to visit your site/blog
as far i concern twitter is perhaps a good thing,because the people at facebook are searching for fun, but ad twitter they all are watching for something else.