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I would say both Facebook and Twitter! Use both these platforms and generate more traffic to your website!
I had my best success with StumbleUpon 10k+ visitors in one day and consistent traffic thereafter. The content of your site matters the most when deciding which social network to use to gain the most traffic.
We have fan pages aswell as our own FB page and always found that you will get the best targeted traffic there rather than anywhere else, however doesnt mean we will ever stop researching and checking out other sites for potential traffic too..
I have a facebook fan page and I can proudly say that 10% of my visitors comes from the fan page. I only have 65 friends