I know that (1) because Snowden's documents say so and (2) because I happen to have met one of the people who worked on one of the communication systems involved in PRISM at one of those companies and implemented the DITU integration at the time indicated in Snowden's slide -- and has the FBI T-shirt to prove it.
The New York Times interviewed several such people and got it right at the start (http://mobile.nytimes.com/2013/06/08/technology/tech-compani...). Greenwald still hasn't gotten it right.