Their log in to your account is done in secret, and once logged in, they can access your private messages, photos, video and restricted posts.
Further, Facebook applies a double standard: if one FB employee accesses the account of another FB employee, then that employee is notified their account has been accessed. Ordinary users (the peons?) are not given a similar notification.
Source: Facebook has a backdoor to your account, and you’ll never know if it’s being used – BGR