An interview claiming to be with a facebook employee discusses a few things that you probably were hoping didn't happen. Here are some choice quotes from the article
Rumpus: Have you ever logged in to anyone’s account?
Employee: I have. For engineering reasons.
Rumpus: Have you ever done it outside of professional reasons?
Employee: I will say, when I first started working there, yes. I used it to view other people’s profiles which I didn’t have permission to visit. I never manipulated their data in any way; however, I did abuse the profile viewing permission at several initial points when I started at Facebook.
Rumpus: You’ve previously mentioned a master password, which you no longer use.
Employee: I’m not sure when exactly it was deprecated, but we did have a master password at one point where you could type in any user’s user ID, and then the password. I’m not going to give you the exact password, but with upper and lower case, symbols, numbers, all of the above, it spelled out ‘Chuck Norris,’ more or less. It was pretty fantastic.
Rumpus: Do you think Facebook employees ever abused the privilege of having universal access?
Employee: I know it has happened in the past, because at least two people have been fired for it that I know of.
Rumpus: What did they do?
Employee: I know one of them went in and manipulated some other person’s data, changed their religious views or something like that. I don’t remember exactly what it was, but he got reported, got found out, got fired.