I respectfully disagree. It's a simple message passing system, of which there are a ton of examples. This isn't rocket science.

The problem is that where once it would have been written in c as a server, it's now written with big databases and 'popular' languages - so it's harder to scale.

It would have taken longer to get working in C, which would have dramatically increased the probability that the project would have gotten killed before it saw users (it sounded like a pretty stupid site to me at first!), and it would be harder to rev the UI and features as often as they did before they hit the scaling problems.

But a message server in Ruby with Eventmachine wouldn't have been that hard, would it?

True. IBM have basically solved messaging with MQseries running on OS/390. All that remains is do that, only cheaper.

Proprietary message queues are a huge problem (and a huge business).

There are better ways: http://en.wikipedia.org/wiki/Advanced_Message_Queuing_Protoc...

For a good introduction check out http://google-ukdev.blogspot.com/2008/09/rabbitmq-tech-talk-...