It seems to draw in a wide array of intelligence gathering sources, including the PRISM backdoors into service providers, the ECPA provisions that anything stored on a third-party server for more than 180 days is legally considered "abandoned" and thus searchable without warrant, interception of or querying of cookies as they are being set up or off the device itself, FaceBook chats (off their servers), web browsing history, and so forth.
The web browsing history could be worked out either by intercepting and recording the traffic to and from the individual devices to the web servers, or to a lesser extent by recording DNS queries made by the individual devices.
What really interests me is this: what's up with the ring of "locations" for X-Keyscore around Antarctica? Communications satellites? Intercept stations on ships? Or … are they watching for signs of the rise of Kaiju?