I have to say stumbleupon since it drives most traffic on my websites. Facebook is great too but didn't work too much on that and it's not better than stumble yet.
Facebook is my number one favorite Social Networking as of now. It's very entertaining and I love to use it on my phone. Plus the backlinks for the website it's totally great.
According to me, Facebook comes first then Orkut, Twitter, Myspace and then other social networking sites.
facebook mostly, also russian stuff - vkontakte.com... and i also cant wait to sign up for myimpression.me