Since Rails is not multithreaded (yet), it seems like a threaded web framework would be a better choice for a Facebook application. (reason being is cuz each Rails process can only handle one request at a time, and facebook actions tend to be slow, because there is a lot of network communication between your app and facebook)
Has anyone used Merb to write a Facebook application? Is there a port of Facebooker (the Facebook plugin for Rails) to Merb?
Have you looked at Starling? It's the server used by twitter to handle their messages. It's a persistent queue server that allows you to delegate jobs to workers.
You can run passenger on Apache which will start up as many Rails instances as it needs up to a certain limit (I think the default is 30). It will also kill them as required, so if you're not getting as many hits as you were 5 minutes ago, it will release the system resources back to the system.
Learn more about passenger at http://modrails.com
Behold, merb_facebooker.
In addition, if you want to use Facebooker directly (like for a desktop app,) just install the gem:
gem install facebooker
We've used merb_facebooker in one of our projects (Rock the Vote), and it worked out pretty well. Testing Facebook apps is quite annoying, as you don't have control of the middleware, so watch out for your expectations of the FB API and make sure you validate as much of them as possible early in the development stages (not trying out all the things we needed to do with fbML early on brought a few headaches).