Humm,
History. (Mostly because I still think in an AIM sort of way about this)
You can have AIM Name or a Jabber iD (we'll forget about Yahoo for now).
You can add Buddies as plaid Screen Names or Jabber IDs.
In all cases these are held at the Server you are using.
From iChat/Messages you can highlight a name/ID in the Buddy List and then do a Get Info (CMD + i keys together)
This brings up the Info Card and will tend to show the Address Card info.
If you Add a Real (First Name, Last Name) then this will be added to the Contacts App/Address Book.
(or you can open up the Address Book/Contacts App, create a Card and Add the Jabber or AIM Info on to the card).
AIM have a way of describing this.
Buddies are "Food"
"Food" is kept in "Feedbags"
The Buddy List is one Feedbag and the Address Book/Contacts app is another.
So the Buddy List could have "cereal" and the Address Book has "Bran & Oat Flakes" if you like.
Once the Address card has a AIM name or Jabber ID linked to the Real Name then the Buddy List will display the Real Name. (You see Bran & Oat Flakes)
Now the Contacts app in Mountain Lion has various ways you can Sync the info.
The most basic is to use iCloud and it will reflect changes made on the Mac or the iPhone across the devices linked/Synced with that iCloud ID.
My Contacts app shows an iCloud option (enabled)
An On My Mac option (Not enabled but offering to Sync with Google)
and a greyed out Inactive Yahoo option (I have used the Yahoo for Mac app to test before).
I would guess you either have a Facebook option of the ON my Mac version has an option to sync with Facebook.
At a point you are showing All Names you could turn the Sync option Off and rely on what is stored on the Mac.
You may have to consider what any iOS device is doing regarding Address Book Syncing.
This will only be a work around though.
10:00 PM Wednesday; August 28, 2013
iMac 2.5Ghz 5i 2011 (Mountain Lion 10.8.4)
G4/1GhzDual MDD (Leopard 10.5.8)
MacBookPro 2Gb (Snow Leopard 10.6.8)
Mac OS X (10.6.8),
Couple of iPhones and an iPad
"Limit the Logs to the Bits above Binary Images." No, Seriously