It's not just the ads, but all of the Trackers that websites load up to track your online activity. I use the Ghostery Chrome plugin also and have it block all of these ad platforms and trackers. It also shows you which trackers are loading for any given page and allows you to block some, or all of them. For example, on this page alone, 21 individual trackers attempted to load. Some pages have more, and some have less, but they are there to both keep a tab on what you are doing so that that information can be shared back to the various trackers to help build your online profile so that ads can be better tailored to you.
I have been on some sites where over 100 trackers attempted to load. Keep in mind that these trackers load behind the scenes and you don't even know that they are there. The only visible ones are the ones that serve ads. Install Ghostery and surf some of your favorite websites and you will be amazed at how much "third party" stuff is loading.
Anyway, just thought that I would share since this is a topic that is getting more and more attention.
66 Mustang Coupe
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