Privacy is a topic that I closely followed since early 2000s. Before we were addicted to free email accounts, the nascent ad-supported free Internet services industry left a lot of questions unanswered with respect to how they collect data and what they do with it. After more than a decade, a Netscape invention called browser cookies is at the forefront of privacy battle.
If you, like so many other concerned netizens, are using a privacy enhancing browser plug-in such as ABP, Ghostery, uBlock, I have a disturbing update: your effort to circumvent all this unsolicited tracking behavior on the Internet is an exercise in futility. There are new generation of persistent tracking techniques employed by the ad companies to slice and dice all unassuming web citizens of the world by some cunning methods.
One is called canvas fingerprinting, which in essence, is an abusive use of a feature, Canvas API, in modern browsers that track subtle differences in rendering of text or other patterns on the browser window. While, by itself, it's not enough to uniquely identify a person based on its machine's graphical processing capabilities, coupled with other information such as installed plugins, screen resolutions, Javascript capabilities, etc, it does not take much to find out who exactly is browsing the site (Using less that 10 different feature vectors, it was shown to be possible to uniquely identify someone).
Other techniques are even more sinister, because they restore cookies deleted previously and share that information across different sites by something called cookie syncing. Historically, when a user clears his cookies and restarts browsing, the third parties will construct a new history graph. Since these graphs belong to different tracking IDs (or user ids), trackers cannot associate the individual's history before and after cookies were cleared. However, if another tracker respawns a particular cookie, parts of two history graph may be linked by a node, thereby identifying both history graphs with you.
There is a lot at stake for online advertising industry. Therefore, they have the wherewithal to explore any and all avenues to make sure that constant revenue stream continues to flow.