I think browser Notifications help drive these attacks. How many web sites do you visit that offer a pop-up that says the site would like to send you Notifications? You click Allow and suddenly start seeing Ads popup in your Notification area, not a site notification but an Ad.

I had a user show me one of these Notification ads just this week, telling here that McAfee found a virus and click the Ad to remove the virus. We do not even use McAfee, it was a straight up attack ad. Thanks Chrome!