How does Facebook manage all the user passwords?

Hello! I have always looked at big systems like Google, Facebook etc and always wondered how they store so much data in such an efficient manner. Storing data is jus one aspect, keeping it secure, I think it is more important than anything else. I don't know but I really want to know how Facebook stores all the passwords of their users in a database. I mean their should be some dedicated database or something like that which is fully secured and where all the passwords are stored and retrieved all the time, right? Can someone elaborate more about how Facebook stores and used the passwords of its users?
