Or it can just be that the more +1's you get, the more links you get, which boosts the rankings. So it may just be an indirect correlation.
Also I don't see a correlation in my blogs between more retweets and more Google's +1s. It is most probable because my blogs don't have many visitors and didn't cross the charm, but I expect search engines like Google to leverage the long tail instead of smashing it.
There's some crossover between a very few platforms, e.g. posts which get a lot of InShares will also likely get Tweeted a lot. It's quite rare, though, in my experience.