Facebook and Twitter just help Quora verify your identity, while also ensuring new users have nice profile pictures. They still have to create their own unique account for you, which is why they want your e-mail.
I felt like I got duped into revealing my Facebook account for no reason. The page indicated that I'll 'save time' but as far as I can tell I didn't save any time at all.
Quora uses your Facebook friends graph to determine who you'll initially follow on Quora. It's been critical to onboarding new users and showing them relevant content immediately.
