I'm a cofounder at CollegeAI.com. Our users can sign up via Facebook, Google or Email. Here are our stats: Google: 58% Email: 23% Facebook: 19% Most of our users are around 18yo.
I noticed that Facebook is listed as the first option with a graphic button, whereas e-mail is listed last with a text-only link. Despite this, e-mail has attracted more users than Facebook. It’s broadly a similar story for other posts here with stats listed.
In my view, this mostly negates the claims above that people who prefer to login with e-mail can be reasonably ignored.