I thought Google could only track you if you were logged in? I was constantly logged into Google+ so everytime I did a search for something it was following me. The thing that really pissed me off was wanting to place a comment on You Tube and them not letting me until I'd updated my settings to how they wanted them. I still have a You Tube account that I set up in 2006 so have gone back to using that and I use my Hotmail account to log into that. So far there have been no pestering pop ups.
Here's something that people don't realise. Each computer system and person have a unique 'signature' you can build a profile of a person fairly easily and to a degree it's accurate enough to build a tangible profile without that person having any engagement with the service like giving sensitive information.
Consider this you visit 5 sites, you buy an item, you research that item on four other sites. You have left a data trail for the advertising analytics. Google can get this information and compile it through their mail servers if you use Gmail, as they scan those e-mails for keywords. Combine all this you have left a data trail bigger than the Mississippi. Further stil if you use your Fb (which also has a shed load of analytics built off your likes, comments, posts and profile) then Google can scrape your profile for details if it's public (or any other website/profile you may have). You can see why people are clamouring for privacy especially in light of the NSA leaks.
What people don't realise it's too late, the horse has bolted. If you're on the web you can be tracked regardless if you use DarkNets, VPNs or anything else to try and obscure your behaviour as numerous research papers have shown. Google has 15 years to build profiles of targets and metrics. They want to make that data even more valuable by converting that data into actual people. By using G+ and attendant services you have just done that. Fb does this all the time. However you do have a choice, and it's usually against the services terms and conditions, it's called data poisoning. You make minor changes to your identity. Do it enough times then it becomes harder to build a valid profile that's saleable. It's also good practice to prevent Identity Theft if a profile or service is compromised. People are willing to sell their details for free services and has been proven time and time again. Facebook, Twitter, Google, Bebo, MySpace and any company that makes from advertising or social engines all make their money from selling your data. There are a few exceptions Apple and Microsoft because they now both have Devices and Services to make their money from. Out of all the consumer tech companies they are probably the safest to work with. I say safe in a very loose sense, but unlike Google and the rest they do have a vested interest in providing a modicum of privacy to it's users.
Of course another choice is to drop off the grid altogether but this gets harder and harder to do every year.