Their main feature is privacy, so if news came out that they were actually tracking users, it would pretty much destroy the business. Given that, it would make @yegg really vulnerable to blackmail by employees.
Again, I advertise I'm not sharing data and everyone buys up into it without verifying. I have asked friends at Facebook about sharing data with NSA, their response is FB is very transparent and someone would have seen it. So it's very possible they share data and their employees don't know.
I hate to use this phrase, trust but verify