The Federal government has the authority to create regulations, create laws, collect taxes, send people to your house to enforce those laws, remove/restrict your rights, sentence you to a prison term, send in armed officers to take down civilians groups viewed as dangerous, etc etc all the way to declaring full scale war. And, perhaps most importantly, the government has the authority to coherence third parties to cooperate in information sharing. Google will never be able to force Facebook to hand over their data, but the Federal government can force both of these parties to hand over their data to them.
The catch with having a monopoly on force is that your hands are supposed to be tied by the will of the people. There is a tremendous and intentional asymmetry in power. This necessitates an equally tremendous system of transparency, accountability and oversight.
I think once a day I hear "people willingly give all there data to facebook, why do they care if the NSA is listening", people need to understand what "monopoly on force" truly means.