Facebook if you are willing to pay to boost posts and your page. If not then it is almost useless. They've made the algorithms for business pages so difficult to get any of your content seen, you would be better off posting your links to your personal page as far more people would see it (on their news feed.) Instagram and Twitter not yet like this, so you can get more organic traffic from those two. G+ is supposedly good for SEO but I can't see its benefits. I just repost my content from Twitter there (via buffer) and include my website link in the profile.
Facebook is most effective social media source for marketing. You will get group basic traffic source from facebook. This is most used social site. From Facebook page you can advertise and reach your desired traffic and you can sale easily from Facebook.
I would like to suggest Facebook as most effective tool of social networking.Facebook is a social site which allows users to connect worldwide.Through facebook anyone around the globe connect & share with friends.You can post contents , like them , invite friends & also share hem with your friends.You can advertise your business on facebook & increase your reach to your customers& grab their attention.
I hope this will help you
Google+ is not considered as a social media site but a search engine. If you make that list, Google+ will surely have the highest traffic.
For the social media site, Facebook and Tumblr are also great, but their traffic are unstable and not as high quality as Quora. I highly appreciated the effectiveness of Quora despite low traffic.