Google's Chief Architect, Yonatan Zunger in this post:
https://plus.google.com/u/0/+YonatanZunger/posts/huwQsphBron notes his suspicions that the second slide (
http://www.washingtonpost.com/wp-srv/special/politics/prism-... ) subtly explains how the data collection works.
The way I see it is that by extracting data via man-in-the-middle (subject to HTTPs etc..., discussions elsewhere), the NSA and others could target packets that fit certain profiles of data, such as Googles, Facebooks etc...
