Hi,
Sorry for the delay as I don't tend to look for iChat or Messages threads outside of Mavericks, Mountain Lion or iChat Communities.
Google decided that their Messaging service would be run a Jabber server.
You could add this to iChat in iChat 3 but due to Google not following other Jabber and XMPP server naming protocols you had to change the Server name manually.
In iChat 4 Apple added a separate Google Account creation spot that got around this.
This continues in Messages.
To explain further most servers follow the account name or Username @ Server name.
So if I joined the Jabber Australia server (@jabber.au.com) then I would be ralphjohns@jabber.au.com
In most apps when I type in my Jabber ID the account creation process strips away my user name to find the Server name.
The issue with Google is they used email IDs which in turn could be either @gmail.com or @googlemail.com so they created a server called talk.google.com
A Google setting showing the server name
Another Jabber Server Name.
Facebook Chat is also on a Jabber server but their "registration" for this creates a separate Jabber ID for chat which is in fact separate for the regular Facebook ID.
8:42 pm Tuesday; July 1, 2014
iMac 2.5Ghz i5 2011 (Mavericks 10.9)
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