Facebook is most attractive and effective channel for customers. Facebook has lot of real users and more popular.
If you are asking for just increasing traffic than you should go with Facebook but if you want to increase quality traffic than you should go with Twitter or Google+ because on these sites you can get more professional persons than the Facebook. Their are more sites you can chose like Pinterest, Stumble etc.
Well, I think Facebook, Pinterest, Google+ and Twitter are the most pop lour social networking channels.
It depends on your purpose. The ones that people commonly use are excellent for reaching a vast number of people. Smaller, more niche-oriented sites might help you target specific groups better.
Seriously thinking, I think Forums is best place to attract new customers. Unfortunately, most of people still use FB for dating sites.