At Kera we're only limiting users to people with google accounts so it's interesting for me to read: "completion rates as low as 50% with forced Facebook login

At Kera we're only limiting users to people with google accounts so it's interesting for me to read: "completion rates as low as 50% with forced Facebook logins, while another one allows signing up with different methods (username/password, Facebook, LinkedIn) claims completion rates of 90%" Do you have more information on this?