ActionController::Caching ... expire_page takes a url_for and tries to remove the file from disk.

Unfortunately File.exists?(page_cache_path(path)) fails since http://apps.facebook.com/.... etc isn't found on the local
file system.

It's being called with

self.class.expire_page(url_for(options.merge(:only_path => true, :skip_relative_url_root => true, :action =>
action)))

-- perhaps your url_for could use one of :only_path or :skip_relative_url_root and not do it's magic then?

See http://ap.rubyonrails.com/classes/ActionController/Caching.html if you want to test out.