Traffic for a website is like blood flowing through the veins: without it, the body dies. The challenge is that the blood will recycle, but social media is on a one-way trip if everything works. The only reason the website exists is to receive traffic, but social media can be considered a tool for delivering a message, and if that is all it does, it is indeed a one-way trip. The better use of social media would be to engage with an audience by inviting responses and comments.
The most successful uses of social media in driving traffic for this website that I have used are LinkedIn, Pinterest, Twitter and Facebook.