[0] https://en.wikipedia.org/wiki/National_security_letter
[1] https://en.wikipedia.org/wiki/PRISM_(surveillance_program)
However, these large firms have enterprise-wide security and too many people would notice the vacuuming of data for this to be done by single agents. So that would require secret court order and secret laws, as we know existed a few years ago.
So no doubt you have some level of secret agency access but exactly how much is difficult to say. Remember these are companies operating globally and it's in their interests to not be seen as mere extension of US intelligence and foreign policy but at the same time these agencies can very persuasive, etc. etc.
https://en.wikipedia.org/wiki/War_Is_a_Racket (1935).
History doesn't repeat but it does rhyme.
It seems to be the natural state that centres of power co-operate with each other lest they lose their power.
Churches with Kings, Corporations with Government.
I'll see if I can find the slide that articulated the issue.
The other part was "Do what we tell you, or you'll be Joe Nacchioed"
In a 2013 interview, Marissa Meyer made it abundantly clear this is why Yahoo "voluntarily" joined PRISM. One can assume the rest were similarly influenced.