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0x0 is saying that when you receive the password you can do 3 checks. No need to invert the hash function there.

Also, inverting a hash function is impossible (the size of the range is less than the size of the domain). Finding a collision, on the other hand, is not.

Ah, yes. Of course, thank you.

Facebook's servers get to see your plaintext password for a fraction of a second every time you log in. That's their opportunity.

Hell, wasn't Zuck infamous for using the mis-entered password of users to break into their email accounts at Harvard?

Even if that tale is apocryphal, the underlying moral is true - your security is greatly undermined if you reuse your password (or key) across sites - any malicious site operator (or even an honest one that has their security broken) will expose you.

Thanks for asking this. I was confused too.