Most of these use encrypted communications. NSA may be able to crack specific communications, but they can't crack all of them from Google, Skype, Facebook, iMessage in real time.

Getting access to it "voluntarily" makes thing so much easier. It says in the slides the PRISM program only costs them $20 million a year.

I wager they have control of many root CAs. They could literally MITM any connection they want to.

This is the best guess if you take both the leaked documents and the companies' denials as accurate. They can use a real prism to duplicate the fiber traffic before/after Google/Apple/Facebook's servers and their root certs to take a peek within.

But those companies have data centers all over the world. It's not like all data goes through one pipe ...

Do you have any more information about how viable this would be? It's a threat I hadn't considered before.

They're definitely doing the fiber portion, see:

http://en.wikipedia.org/wiki/Room_641A

If the NSA were specifically interested in YOUR communications, MITM would be the least of your problems.

Most importantly, if they can crack and get the information they most likely can't use it unless there is an imminent and grave danger to the government's ability to stay in power. Because then using it would reveal their abilities.

Now there is a system that can be used for example. That system is once NSA identifies the list of suspects they forward that record to a another agency (FBI?) who then is in charge of finding other evidence, which could have plausibly be found anyway, and prosecute based on that. They can never release the initial reason why they got "interested" in someone as that might reveal the abilities of the NSA.