I am using Devise with Omniauth to have users sign into my app with Facebook. I used the Railscast tutorials to get it up and running.
If a user is already a member of
Add :password => Devise.friendly_token[0,20] when creating a new user from facebook omniauth.
I believe Devise is expecting something in the password field to create a User. Since there is no password when doing facebook oauth (not on your app side at least), you just need to create a dummy password as show above.
See this for more info: https://github.com/plataformatec/devise/wiki/OmniAuth:-Overview