What kind of ads? No, Yahoo! does not do that. They may send some internal ads about Yahoo! to individual Yahoo! members, but they do not usurp your address books. What might have happened? Well, you might have had some malware that did it.
Have you ever sent a single email to your entire address book? Malware of various sorts could capture the address book under some conditions.
Has anyone else used your account?
Okay, let's see what other suggestions come in.
