What is this string? And what could it mean?
For those viewing raw logs, or using third party tools (like Sitemeter, which I use in addition to Google Analytics, Quantcast and Icerocket, just to overload myself with data), you can see Google+ referrals having the following string:
http://www.google.com/url?sa=z
If you can track how many times visitors come your way with that particular string in their referral, then you can know just how much an impact Google+ is giving you downstream. For me, despite reports from others, I've never gotten much traffic at all from Facebook, and Twitter tends to like a specific type of story, inconsistently. FriendFeed, once my 2nd highest referrer, is practically gone. You get what you give in the social networking space, so no doubt if you participate in G+ (as the cool kids are calling it these days) or you make it part of your flow, you can see it bringing visitors your way.
So watch for that string if you care about this stuff. There's no Gmail excuse this time to obscure transparency.