How can LinkedIn know about my Yahoo contacts?

Question

I signed up to LinkedIn using a Yahoo account. I confirmed the address from their email message, but I refused to allow them to access my address book. So the site showed:

Confirm your Yahoo! account.
Click below to sign in with Yahoo! and verify your email address.
We'll let you know if your Yahoo! contacts are on LinkedIn even if you don't add your address book.

How can they do that even if I refuse address book access?

Answer 1

They might checked your e-mail address in other users address book (people who did agree giving their addressbook).