Indeed, with all the Facebook sharing buttons around the web they have more than your bookmarks too, they have the equivalent of uploading your browser history each day to them. With current machine learning technology, let alone improvements over the coming years, they'd know more or less what someone would know if they stood behind every time you were browsing for the last few years.

Even if you were to cancel your account you'd turn up in photos of parties on other people's accounts and they'd follow pretty well you after you'd left too. With their dodgy history, Zuckerberg's contempt for his own users as evidenced by his quotes, the idea that this won't be abused is laughable. Given national security letters (NSL) it's almost certain the intelligence community has access. Hell you could hit a mid-level admin at one of their data-centers with a NSL and the top level management and legal wouldn't even be aware that there are outside entities with a connection to their database. That's to say nothing of the extra-legal capacities of these people.

While people like to think that it's more or less anonymized clusters and demographics that are being passed to advertisers to target ads better - not a terribly scary thought, just more relevant ads - I think we need to be far more cautious.