Here are the set of advertising trackers used by a local newspaper that is part of the USA Today Network, owned by Gannett Communications, the largest newspaper publisher in the U.S.
65 of them on a single page!
Separately, I paged down through my Twitter feed and after a few pages, a total of 119 trackers were blocked – so far. If I continued paging through the feed, even more trackers were blocked.
Trackers could be used merely to determine how many times an ad has been displayed, or how many times seen by the same browser, or to gather information.
In reality, they are tracking as much as possible and stuffing it into giant databases. If you visited 3 web sites that displayed DoubleClick ads, as an example, they can link together your visits between those web sites. Over time, they can discern your interests (do you visit many computer or photography related web sites?) and probably your sex, age and income range. This data is extremely valuable to advertisers as it enables them to target ads right at your interests!
You can block many trackers using browser plug ins or privacy browsers like Epic. You can and should frequently delete your browsing history and “cookies” to reduce the effectiveness of these data collections.