Breach in facebook security?

...why? they are 2 unrelated companies using unrelated systems

it's also not hacking, your password was guessed or leaked from somewhere else

Even though I doubt the hacker knows you use steam unless you have clues on your facebook is always encouraged to change everything (passwords, email passwords or even the email, logs name, etc.) if you haven't done it yet. If you do that there is definitly no way the hacker can access your steam account even when owning your facebook account.