There’s a fascinating post at ReadwriteWeb by Scott M. Fulton III about the effect “social signals” such as posts by people within your Google+ Circles, has on search results. It is not an easy article to skim :) Here’s the conclusion:

It is obvious from our test so far, which spanned a 48-hour period, that there may be an unintended phenomenon of the infusion of social signals into all Google searches: the reduction in visibility in search results of the original article that generated all the discussion in the first place. This may have a counter-balancing effect on the popularity of any article…