Well that's great, but it's also stupid. Companies like Google and Facebook have hundreds of high-level engineers staring at all levels of their system all day long, trying to find out where their microseconds have gone. And these people are responsible for umpteen billions of dollars in capital expenditures every year, and responsible for capacity planning and so forth. The theory espoused at the link you posted requires that all of these people are either not smart enough to notice that an external entity is using their resources, or that these people, who I would point out are largely not Americans, are in on the conspiracy, or, finally, that the NSA is capable of pulling off their surveillance without having any detectable impact on production CPU, memory, storage, and networking.
It is also, as far as I know, illegal to hack or disrupt computer networks in that way (even for the NSA). If they had warrants giving them access to information they wanted it would have been overkill to do something like that, risk getting caught, and now have to go to the company for intelligence cooperation in the information.
and if the NSA should break the law that makes it illegal to hack networks in this way, what would be its punishment? Do we send the NSA to jail? Do we fine the NSA? Who do you think pays the for the lawsuit (punishment, legal team, etc) when the NSA breaks the law?
I had the same thought as you, which is why I posted the skeptical response "... All without telling anyone or getting noticed by ops people etc."
To anyone who notices, there's always "shut up and not notice." But there's also "oh that rsync you see there is for our geographically redundant backup facility" or whatever -- in other words, dissembling.
These are highly implausible scenarios.