Originally, I intended to finish my comments on Facebook with the last part of my series. However, since I just read the very interesting articles Documents: U.S. mining data from 9 leading Internet firms; companies deny knowledge in the Washington Post and NSA taps in to systems of Google, Facebook, Apple and others, secret files reveal in the Guardian, I changed my mind. According to them the NSA and the FBI are tapping directly into the central servers of nine leading US Internet companies, extracting audio and video chats, photographs, e-mails, documents, and connection logs that enable analysts to track foreign targets. One of these companies is Facebook.
This finding corresponds with the experience of a trustworthy German acquaintant of mine. She travelled to the US and was checked by an immigration official at an airport. During the interrogation she mentioned her Facebook account. Without knowing her password, the official was able to access her account. He tried to check there if her answers were correct. I do not know other cases, but apparently the US immigration authority can tap into Facebook accounts as well, possibly with the help of the NSA and/or the FBI. If you had similar experiences, please add a comment here or let me know.