It depends on the area or niche for which the user is willing to use.
LinkedIn is best for business and profession, Instagram, Facebook, Twitter are used for making new friends and sharing stuffs with friends.

Well your question cannot be answered directly. All social networks bring back traffic to your website
But what will ultimately work for you depends on what you have to offer, what kind of image do want to portray for your company, what is your target audience etc. What works for one might not work for the other. From my personal experience, Facebook worked for me when I was working for a webcasting firm.
If you are unable to analyse and finalize which channel might work for you, you can enroll the services of a digital marketing company, who can help you with the same.

Facebook, Twitter and Google+ is best Social networking websites for Increase traffic.

You have to know where your targeted audience spend their time on the internet, from there you can now focus on what social channel you are going to boost your ads in order to gain lot of traffic from your desired audience. So know first your targeted audience, then take some research where they usually spend their time online and then go with your social media marketing plan.

Facebook twitter and Quora are best for increasing traffic.

Facebook,google+,twitter,StumbleUpon, Digg

Facebook, Twitter, Google+, LinkedIn is the best social media channel for traffic.

Facebook, twitter, Google+ for the website traffic,

Don't underestimate the power of a pinterest.

Linkedin
Pinterest
Reddit

have you tried pinterest

Facebook
Twitter
Google+
Linkedin
Youtube
are best social channel

Facebook
Twitter
Google+
Linkedin
Pinterest

One great way to do this is to share your content on social media more than once. ... Like I said, promoting content more than once actually is a good thing, as long ... Next day – Messages are shared again on the appropriate social channels ...

Facebook, twitter, pinterest,... you can find some list on google, there are so many way to do that.