Memorizing a library card number

i memorized my library card # right after the first time using it to sign in to library's computer, and now I use it as the password for some accounts.
I've also memorized many 9-10 digit numbers, there's this chinese software called QQ which is similiar to msn. except the person's accounts are 9-10 digits numbers, so I have to memorize the numbers to add people as friends when I go home.