Social shares from the major platforms (Google+, Facebook, Twitter, Pinterest, LinkedIn, Reddit, Stumbleupon, Delicious) should be part of your normal link-building activities. Unfortunately, it's impossible to say the exact impact that they will have on your rankings.
For pages I really want to rank, I will intermittently buy the social signal package from the SEO Armani BST in the marketplace, which includes shares to the aforementioned. No clue if it helps or not, but it helps me sleep at night and only costs $5.