https://www.theguardian.com/world/2013/jul/31/nsa-top-secret-program-online-data
XKeyscore: NSA tool collects 'nearly everything a user does on the internet'
XKeyscore gives 'widest-reaching' collection of online data
• NSA analysts require no prior authorization for searches
• Sweeps up emails, social media activity and browsing history
• NSA's XKeyscore program – read one of the presentations
https://en.wikipedia.org/wiki/XKeyscore
You could read anyone's email in the world, anybody you've got an email address for. Any website: You can watch traffic to and from it. Any computer that an individual sits at: You can watch it. Any laptop that you're tracking: you can follow it as it moves from place to place throughout the world. It's a one-stop-shop for access to the NSA's information.
… You can tag individuals … Let's say you work at a major German corporation and I want access to that network, I can track your username on a website on a form somewhere, I can track your real name, I can track associations with your friends and I can build what's called a fingerprint, which is network activity unique to you, which means anywhere you go in the world, anywhere you try to sort of hide your online presence, your identity.