Don't let users sign in with e-mail. Just let them use their facebook account. That's the best way to do it. Try to sell it as a feature, something like: "Are

Don't let users sign in with e-mail. Just let them use their facebook account. That's the best way to do it. Try to sell it as a feature, something like: "Aren't you tired of filling forms with your data over and over again? We are too! Use your facebook account."