So they know "John Smith" whose friends with "Joe User" on Facebook, has an email address for "Joe User" that is different than the one listed for "John Smith"'s account; now they have both. That's part of their business.

Personally, I own a domain. Every retailer, website, etc. I interact with gets their own email address. I have no doubt Facebook knows some of them, and can maybe even connect some of them to my profile -- but it's not certain.

P.S: I don't even use facebook myself.