Sklivvz objected to the answer (and someone - presumably him - downvoted it) based on:
This is not an answer, it's unreferenced and it's wrong. You can only access publicly available information on one site. This you can do by browsing facebook easily anyways. The claim is that IAs can get to publicly unavailable information on all major social networking sites - in a similar fashion as the NSA tapping of telcos
"publicly unavailable information" is a critical piece of info to how to answer this claim, and was missing BOTH from the quote source (the interview), AND from the posted question's clarification.
The question and the interview are 100% unspecific on what they mean by "major SN sites". I provided information to the one that has a lion's share (Facebooks - 600 million active users as of 2011 as per Wiki, compared to 35 million for MySpace).
Other than lack of the links (mostly since I was still editing the question), I feel that the rest of the objections to the answer were not warranted given what info was present in the question.
P.S. The question used as example is just that - an example. I am asking about generic concept of punishing the answer for lack of detail when said detail was not present in the question.