[incision]

>Firstly, you don't need Facebook to login.

As I recall, Quora did require a Facebook account for a while [1]. I imagine that someone initially put off by that requirement isn't going to be particularly inclined to continually check back to see if/when it changes.

Also, since removing the Facebook requirement the means to sign-up with an email address has gone through several iterations of varying unobviousness.

1: http://www.quora.com/Why-did-Quora-decide-to-stop-requiring-...