I posted this a week ago - I discovered the problem - it appears that WordPress multisites get fooled and spit out an error if you try to enter a full URL in any user field. It will work in your main site but not in any subdomain blogs you may have.
I looked in the error log and found the following:
[Sun Jan 20 12:12:27 2013] [error] [client 126.96.36.199] File does not exist: /home/mysite/public_html/403.shtml, referer: http://subd.mydomain.com/wp-admin/user-edit.php?user_id=2&wp_http_referer=%2Fwp-admin%2Fusers.php
So in summary: I created a Facebook user field in WordPress. I can store "https://www. facebook.com/username" into that user field only in my main blog: http://www.yourblog.com . But WordPress will spit out an error in the userdir.yourblog.com multisite of my wordpress multisite install. This is happening on all my sites and I'm hoping someone can lend some assistance as it seems to be a bug?