Got this notification when I opened Chrome when coming back to my desk after lunch.
"We changed our privacy settings to allow us to snoop on what you're looking at and shove you ads accordingly. Feel free to opt out, but we'll probably opt you back in when you aren't paying attention."
Just because you don't click on an ad doesn't mean it didn't work though. If you see an ad for Coke you may not click on it to order a case of Coke online right away, but when you go out to lunch maybe you'll fill your cup with Coke.
I mention food ads because I feel they are particularly effective for this type of behavior. You don't need to click on a food ad, but I know I've had a craving for a certain restaurant or food from seeing it mentioned online (whether an ad or just a comment/post) and then gone to get that food for dinner.
Of course, this type of ad result is very difficult to track.