The exhaustive study published by Eric Enge named Direct Measurement of Google Plus Impact on Search Rankings reveals that Google+ doesn't have a direct impact on Google rankings. The social networking platform offers personalization and improves visibility, which can increase the possibility of getting more links. But, standalone, it will not have direct impact on rankings.
Matt Cutts, the Head of Search Spam in Google has also told this time and again.
Here's a video interview of Eric Enge & Greg Jarboe on their study:Google+ Doesn't Impact Rankings Directly!,