RockYou, a California based company, made it possible to authenticate on facebook applications without having to re-enter passwords. In December 2009, it was hacked.

The database containing the unencrypted names and passwords of its 32 million customers was stolen and then made public.
An analysis of the passwords showed that two thirds of the passwords were less than 6 characters long, and that the most commonly used password was 123456.

This list of passwords, sorted by frequency is frequently used in CTF.
On Kali, the file, zipped, is stored in: /usr/share/wordlists/
In the terminal, to get into good habits, the file can be found at: /usr/share/wordlists/rockyou.txt

password list Rockyou: rockyou.txt