It's not conjecture. Just look at any form your fill out, from taxes, to medical, to employment. They all ask for your email address and not for your facebook url.
Taking an average of the various reports about Facbook usage in the US, about 70% of internet users use Facebook at least once per month.
Somewhere near 100% of internet users use email.
As ok123456 points out, there's a reason that you get asked for your email address and not your Facebook ID. Asking for Facebook leaves out a huge chunk of people.
Email is still the lowest common denominator.