It looks like the Yahoo! Contacts API does not return contacts that have been imported from Facebook.
I have oauth-authenticated with the service and used this endpoint: http://social.yahooapis.com/v1/user/guid/contacts;count=10;start=0
The contacts that I had added manually have been returned correctly, but the ones that I imported to yahoo from Facebook were not returned. If I go into a contact and add a field like nickname and then save, then that contact becomes available to the API endpoint.
To check that it is not buggy code of mine, I have logged into my plaxo account and imported my yahoo contacts - I got the same results. Anything that was imported from Facebook did not make it through the API.
Have you tried removing the pagination parameters http://social.yahooapis.com/v1/user/guid/contacts;count=10;start=0 . Just to make sure you are fetching all contacts instead of only first 10. I just tried the same api after importing facebook contacts. I do see FB contacts in the returned list.