Because it's not their information to play with. If I willingly give my information to Google or Facebook, I don't expect the government to get their hands on it immediately and begin interrogating me via proxy.
There have been what... 3, 4 deaths from terrorism this year in the USA? And 6,000+ deaths by way of guns/criminal activity? Doesn't really seem to be worth their trouble to be snooping through who I talk to and who I hang out with.