How do you hack somebody's Facebook account?

[Amanda Durrand]

Without downloading anything or taking any surveys?

[Drizzy]

It's not possible also what you're wanting to do is illegal.

[Earl]

yes it is possible,
and its usually done by guessing the passwords of people whose accounts you want to "hack".
Many people don't bother to pick secure passwords, they opt for something like "pass123", so it doesn't take much time to hack their accounts.
As for legality you can always argue that you were merely trying to guess your won account's password and by mistake opened someone else's. So you're good legally.
Happy hacking!