goodroi - 12:59 pm on Jul 5, 2013 (gmt 0)
Before jumping to conclusions it is usually a good idea to thoroughly test assumptions. How have you tested your assumption?
You mentioned your website gradually dropped in SERPs, this could have been due to a loss of backlinks which overpowered any potential ranking boost of Google+. Did you track the link juice during your testing to make sure it didn't change?
Also did you track your competition? It is possible that Google+ did give you a ranking boost but your competition gained much more by boosting links or improving their on-page elements so even though your ranking score improved, you can still lose rankings by being outpaced by your competition.
I am not saying you are wrong. I am just trying to encourage others to do better research and testing so they can discover better insights. From my research I do not see a significant ranking boost from Google+ today but I suspect that will change over time. I still often implement Google+ simply for the CTR boost that I have also observed.