Facebook tracks every web page you visit, even after you close your browser and shut down your computer. The cookie Facebook puts on your machine will log you back in automatically once you open your browser, the the tracking continues. The cookie expires in 30 days:
For this to work, a person only needs to have logged into Facebook or Twitter once in the past month. The sites will continue to collect browsing data, even if the person closes their browser or turns off their computers, until that person explicitly logs out of their Facebook or Twitter accounts, the study found.
Of course FB's use of the data is innocuous:
Facebook says it only uses the data for advertising purposes when a user clicks on a widget to share content with friends.
FB also say the data is anonymized, which is very hard to believe as the cookie can certainly be matched to a particular user. It's the same excuse Apple used recently when they denied tracking IPhone users: While technically the present latitude and longitude of a user are not recorded, Apple has the algorithm to calculate the user's position from the collected data.
Facebook also say they keep the data for just 90 days (duh!), after which they mercifully delete it. This is also BS as they certainly back up and archive this data somewhere, lest the vampire squid and “investors” deem the FB management stupid and demand changes.