From this firsthand account inside Facebook HQ:

• One of the most troubling revelations in the anonymous interview is the claim that any Facebook employee could log into any member account with a single master password (which was some derivation of Chuck Norris — not so funny in this scenario). The shadowy interviewee also said that various employees (at least two of whom were terminated) were caught inappropriately using that password to gain access to accounts.

More: Anonymous Employee Reveals Ugly Details of Facebook’s Inner Workings.