Quora has publicly stated after they added a non-Facebook login option, that registration went up a lot. They have openly questioned that call. (Though I have not seen them seek the same feedback about the real-person-only policy).

I think a lot of people who really like Facebook, still don't want it to be following them all over the rest of the web. So FB's high adoption doesn't necessarily mean people want it to use it everywhere.

I never said low, I said lower. :)

Quora… how popular is it really outside of the tech community? Do we really have any idea about what its adoption rates would be otherwise? I don't.