Facebook chat is developed in Erlang
Facebook recently announced that their chat system is based on Erlang code running on a large cluster. It confirms that Erlang is a key tool for building large scale applications, especially for social networks.
Like ProcessOne has used Erlang to build its Instant Messaging suite of tools, Facebook is using Erlang to build its web based chat infrastructure.
The system uses a Comet based programming approach. From our knowledge the system uses Mochiweb as its web server to handle the massive number of long polling connections.