Hackers compile massive lists called "combolists"—combinations of usernames, emails, and passwords stolen from various sites (LinkedIn, Adobe, MySpace, etc.) Because people reuse passwords, attackers extract the emails and try them on Facebook. They save the working pairs into passwords.txt or facebook_login.txt .
While this rarely involves Facebook’s actual internal servers, it frequently exposes passwords of users who have registered on smaller, insecure websites. If those users index of password txt facebook login top