There is not a "one size fits all" social media. It depends greatly on the industry you are in and the market you are trying to target. For example, if you're primarily a business to business service provider then you may be better off focusing on LinkedIn.

Facebook and Google+ :)

Definitely it is facebook,twitter,google+...are enough for fatching traffic on your site.

Facebook
Google+
Twitter
Youtube
Linkedin
Pinterest
Tumblr
These Social Site is best for creating more traffic into your Site.

As per my view,
Google Plus , Pintrest , Facebook.
For the seo purpose google plus is most important . We get the good quality of backlink from gplus and pintrest.

Regards,
Sanvari Fashion.

Facebook and G+ are drive more traffic to your sites.

Facebook
Google+
Pinterest
Youtube
Twitter
Tumblr are the Best Social Sites for creating more traffic in to the Website.

For me, it's the Facebook that generates most of the traffic. Then - Twitter

Use different social media sites such as Twitter, Linkedin, G+, Facebook, Instagram etc.
Create useful and engaging content to share on various social networks and interact with your audience.

if you want to engagement you use facebook.

Is Facebook still effective if you can't afford facebook Ads?

Facebook, Twitter, Google+, Pinterest, LinkedIn, and Instagram is the best social site which creates more traffic.

Facebook and google+

Generally we use the Facebook, Twitter, Google+, Pinterest, LinkedIn and Instagram for generation more traffic.

Facebook and G+ are the site which is responsible to drive more traffic to your sites.