A study by Stone Temple Consulting, announced today, apparently confirms that Matt Cutts was telling the truth. ;)
Greg Jarboe of Search Engine Watch interviews Eric Enge of Stone Temple about the study, which Eric meticulously describes in this YouTube video... Stone Temple Consulting Study Finds Google +1s Don't Cause Higher Ranking YouTube - Sep 17, 2013 - trt 18:05 http://www.youtube.com/watch?v=PfNbTzjvRGI
From Eric's description, his was a very carefully done study... for me a model of the kind of care we should hope for. He introduces his remarks with comments about the difference between correlation and causation. He also notes that, in his study, not everything went completely as planned, and there are differing interpretations about the data.
As I interpret the glitches (which were some additional Google+ shares, not anticipated in the original plans)... if anything, they would have driven the data the other way, and they apparently didn't. There were also some controls built into the test, and they seem to confirm that the test wasn't distorted.
SEW's description, posted with the video, summarizes Eric's conclusions...
In terms of discovery, Enge says, "In my opinion, it is highly likely that Google Plus drove discovery of the content."
In terms of indexing, Enge found that Google Plus shares probably drive indexing, as well. However, of the six articles tested (three test pages and three baseline pages), all six articles initially appeared in the Google index 10 days later.
And in terms of ranking, Enge says, "We saw no evidence of Google+ shares driving ranking."
PS: Eric puts a finer point on it, I should add, and emphasizes that test involved non-personalized results, and that, in his opinion, the Google+ shares are not driving non-personalized
ranking. He also emphasized that personalization is part of SEO, and should not be ignored.