facebook: fixed broken contact counting with non-friends

When fetching more than 500 contacts, multiple requests must be made in
order avoid request limits. When friends are retrieved and processed,
the plugin must keep a count of the processed contacts to know whether
or not it needs to retrieve more contacts.

Currently, this counter assumes every contact is a friend, which is not
always the case. This counter needs to advance even when a contact is
not a friend.

This fixes the incomplete fetching of the buddy list when a user has
over 500 friends.

This is a regression introduced by 25bab54.