Well, I've only experimented with Facebook and Twitter recently, and I think both works depending on how you use them to your advantage. I think Facebook works wonders, and you should definitely look into there for long-term users for your website. But, if you are looking for something immediate you should try twitter. You can easily see result faster compared to facebook. Although, results might not be long term. Nevertheless, it all depends on how you use the sites to your advantage like I stated above.