Here's my empirical stance: I've seen great conversion rates on that screenshot, and I've never seen a compelling example of bumping conversion rates through ad

Here's my empirical stance: I've seen great conversion rates on that screenshot, and I've never seen a compelling example of bumping conversion rates through adding a username/password option. Conversely, at the last place I worked, offering both options was the #1 cause of customer support requests. People sign up through both paths, then ask why everything is fucked. We ended up dedicating a lot of resources to a crystal-clear UX to avoid this problem. Finally, when someone signs up through Facebook, you're often able to offer them a better product, because people empirically respond really well to their friends' faces, and everything else that comes with fb. Facebook permissions also promote your business goals like woah. I'm open to more information, but this article doesn't have any. "Giving users only one way to sign in and FB as the only option is going to turn off a large number of users"? I've reread that sentence several times and I can confidently state there are no numbers in it.