Apple, Google, Microsoft, and various other major US tech companies participate in PRISM. [1]
The NSA internally described PRISM as "extensive, in-depth surveillance on live communications and stored information" with examples including email, video and voice chat, videos, photos, voice-over-IP chats (such as Skype), file transfers, and social networking details. Quoting Snowden in regards to this, "in general, the reality is this: if an NSA, FBI, CIA, DIA, etc. analyst has access to query raw SIGINT [signals intelligence] databases, they can enter and get results for anything they want."
Note this includes live communications! So this opens a pretty big question. How? One extremely conspicuous point is that when companies initially tried to deny being involved in PRISM, they all claimed something more or less the same. 'We do not allow any government agency to have DIRECT access to our servers.':
- Apple - "We do not provide any government agency with direct access to our servers"
- Facebook - "We do not provide any government organization with direct access to Facebook servers"
- Google - "We have not joined any program that would give the U.S. government —or any other government— direct access to our servers"
And so on. This is conspicuous enough to have it effectively said that access to the NSA is facilitated through some "indirect" means. The exact technical implementation of this totally-not-a-backdoor is not as relevant as the fact that this totally-not-a-backdoor can be exploited by other actors if the means are discovered. And of course there's plain old social engineering and spy games in play. Or even plain old greed. NSA worker #235,123 gets evidence that the world's wealthiest man, arguably the single most relevant economic player in the world, is cheating on his wife with some rather extreme and embarrassing evidence of such? That's going to be worth a whole lot to many people who would like to use that information against him, and an NSA worker would be just the sort of person to know who'd be interested in it.
The US got very very lucky that Snowden seems to have been patriotic and ethical, but nonetheless his actions emphasize that all the scrutiny and profiling in the world can't really predict what people will do. And as we continue to expand our surveillance state, it means the damage a single disgruntled worker can do continues to grow in proportion.
[1] - https://en.wikipedia.org/wiki/PRISM_(surveillance_program)