WordPress.com hasn't run on FreeBSD since the TextDrive days which was way before Layered Tech, before me and before WordPress.com was open to the public. We are 100% Debian today, but have used Ubuntu in the past. We never really used HAProxy either. I posted about out load balancer choices back in 2008 - http://barry.wordpress.com/2008/04/28/load-balancer-update/