Logging into Card Mania Legends is needed for saving your progress (cards collected and created).
No worries, the site won't post anything on your Facebook wall, nor spam your email. Having an account here is just to keep your data safe and make it available on all your devices.
If you own a Facebook account, then logging in with Facebook is the quickest way to start using Card Mania Legends.
Card Mania Legends will send you a special link which you can use for logging in, so you wouldn't have to choose and remember yet another password.