There is literally no evidence anyone could produce which would prove to anyone's satisfaction that (a) Google, Yahoo, Facebook, etc don't hand over every last shred of data they have to the NSA, the KGB, and whatever China's equivalent is, or that (b) the CEOs of the respective companies didn't personally sign off on every single violation of privacy while kicking puppies. Corporations can't have alibis. Mark Zuckerberg can't say, "Oh, I was in Cincinnati for the last 5 years, and I don't have cell coverage there, so I can't possibly have been involved."
I usually avoid defending billionaires, because they can dry their tears on their giant stacks of money, but so far this is just a he-said-she-said, and if all of the tech companies are saying they're innocent, it's the responsibility of the accusers to pony up some evidence they're guilty. Slide shows are one thing, but if you want to prove to me that the system does what you say it does, we need a document dump: millions of random emails, Facebook messages, whatever that have been illegally accessed through this system, which could plausibly not have been accessed without the kind of far reaching access that the tech giants are being accused of having provided.