The stack is all Python (2.7) using Django (1.3) on Ubuntu (11.04) and it works great. Python is fantastically easy to program and Django does plenty of heavy lifting. We use the ORM in about 95% of the queries and raw SQL in the other 5% (The Django ORM doesn't handle all m2m queries very well).

We use Celery for asynchronous processing and everything is running on Amazon Web Services. We use ec2, RDS, S3, SQS, CloudFront, Route53, and ELB so we are heavily invested on that front.

Yeah. I built the first version using Django but my code has been re-written by our engineers. I'll have our CTO chime-in on the stack.