A new study out of Princeton University highlights Google's extensive use of tracking cookies to keep tabs on Internet users and underlines the reasons privacy-conscious people might want to block them.

In what may well be the largest study of online tracking to date, the researchers analyzed data from more than a million websites. They discovered that Google Analytics, a tool used to analyze Web traffic that integrates with ad-targeting apps, was embedded in nearly 70 percent of the sites. Along with Google, the leading trackers identified by the study were Twitter and Facebook.

As the (tracking) cookie crumbles

A cookie is a bit of code that certain websites attach to your browser. Cookies can be beneficial — they keep you logged into your favorite sites, for example. However, they can also be used to track your movement on the Web, and then serve ads that are supposedly tailored to your browsing habits. In general, such tracking code isn't harmful, but it can be annoying. After you look at one product on a retail site, for example, ads for similar items can follow you around the Web.

More compelling reasons to block cookies also exist. The United States National Security Agency reportedly uses a particular kind of tracking cookie from Google to identify and track people it many choose to eventually hack. Cookies often share the data they collect with third-parties that can then use the information to construct profiles of users. Facebook also uses tracking cookies, and because Facebook already maintains a good amount of user data, additional details from the cookies make those profiles even more complete.

News, arts, and sports sites do the most user tracking, and they're even more likely to track people than pornography sites, according to the Princeton researchers. "Since many of these sites provide articles for free, and lack an external funding source, they are pressured to monetize page views with significantly more advertising," the researchers wrote. Non-profits, universities and government sites are much less inclined to keep tabs Web surfers, probably because they don't depend on ad revenue.

Cookie blockers effectively reduce online tracking

The study did contain some good news: Cookie blockers work. For example, Firefox's third-party cookie blocker is "very effective," and the browser add-on Ghostery successfully blocks certain types of cookies as well, according to the researchers. (All of the most popular browsers support Ghostery, with the exception of Microsoft Edge, which does not yet support any extensions.) The Electronic Frontier Foundation also offers a free cookie blocker, called Privacy Badger.

Some browsers let users tell websites not to track them. Unfortunately, many sites simply ignore the requests, which is why it's a good idea to use a cookie blocker.