We are obsessed about security. (Probably even more than your bank is!) So we use cutting-edge technology such as this "Sign in with Facebook" button, which is called a "federated login".
We never see your passwords. You log in directly at Facebook (and if you're like most people, you're probably already logged into Facebook), and then with the push of a button, you're logged into our site, too.
We recommend following Facebook's Security Tips: make sure your Facebook password is long and unique (a password that you don't use for any other site), and enable two-factor authentication.
[In fact, those same best practices (long, unique passwords plus two-factor authentication) are how you should secure ALL of your online accounts: bank accounts, investment accounts, Gmail, Amazon, etc.]