I deal with a lot of high school students. To them, Facebook is identity. Email is throwaway.
- Many people never abandon their primary identity email.
- But many other people often abandon their email identity.
- You don't want to use a social network as the gatekeeper to *your* site.
- You don't want to have human support for password resets.
Sekrit questions/answers along the lines of your first car's maiden name? Those can be forgotten just as easily as usernames for login.