I mean, can you set up different facebook app id logins per sub domain.
Like the vB one (
www.vbulletin.org/forum/showthread.php?t=25419 ), you set it up as:
[domainx.com]
bbtitle=Domain X Home
bburl=domainx.com
homeurl=domainx.com
catids=1
styleid=1
cookiedomain=.domainx.com
facebookappid=Your_appid_here
facebooksecret=Your_fb_secret_here
[domainx.com.au]
bbtitle=Domain X Australia
bburl=domainx.com.au
homeurl=domainx.com
catids=4
styleid=1
cookiedomain=.domainx.com.au
facebookappid=Your_appid_here
facebooksecret=Your_fb_secret_here