You don't need to log in with your Facebook/Twitter account; I was able to sign up just fine with an email address. It's just one of the authentication options

You don't need to log in with your Facebook/Twitter account; I was able to sign up just fine with an email address. It's just one of the authentication options they provide. (It also lets them import your contacts from those services, which is handy for people who are big into Facebook or Twitter.)