After reading an article about an well-known, internet-based, pseudo-detective company that will sell info about anyone to anyone, I paid a few dollars for their basic info package about myself. I was shocked to find out what they knew, and have no idea where they could have gotten the info, presumably available for everyone.
They knew my last job title and salary (I retired a few years ago), where I had lived recently, the price of my various houses, my childrens' names, where they had lived, what their salary and jobs were, and on and on. For a few dollars more, they would provide much more depth, including court-related records and so forth.
Here's a link to this company, presumably one of many:
http://www.intelius.com
I've read that it's become all but impossible to get mistaken information about a person corrected or withdrawn. There are even companies that specialized in cleaning up mistaken information. I've also read that employers routinely do various internet checks about prospective employees, now considered a matter of due diligence. Thus, internet-based tracking info is more than just an advertising-related curiosity.
I'm sure the law enforcement agencies, both open and secret, use these technologies and more. I read some time ago that the US security agencies scan all internet traffic for their purposes. Assuming this memory is correct, that would take one heck of a computer farm.
I understand that Europe has various privacy and database-related laws to protect their citizens. My understanding is that the USA has almost none.
Perhaps we need to restrict our posting of mating bugs! After all, who wants to be listed in these databases as some sort of sex offender!! (insert faint attempt to smile here)
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There is an add-on for Firefox that stops most of the tracking systems. It's called "Ghostery". You have to turn it on and then for the first few days manually ask it to block the new trackers on web pages. Once this is done, it seems to work OK. I do have to turn it off to access an occasional web page so I am guessing it might have some false alarms.