Gabriel Weinberg via Quora:

Alarmingly, Google now deploys hidden trackers on 76% of websites across the web to monitor your behavior and Facebook has hidden trackers on about 25% of websites, according to the Princeton Web Transparency & Accountability Project. It is likely that Google and/or Facebook are watching you on most sites you visit, in addition to tracking you when using their products.

As a result, these two companies have amassed huge data profiles on individuals, which can include interests, past purchases, search, browsing and location history, and much more. This personal data is stored indefinitely and used for invasive targeted advertising that can follow you around the Internet.

Via Twitter: