Facebook has announced Facebook Messenger, a dedicated app for Facebook messaging. It's already possible to send messages and chat within the standard Facebook app for the iPhone, but this new app hopes to streamline the process by putting the focus squarely on the chat interface.
Messages can be delivered through both notifications and texts, and you'll have the ability to message back and forth with people in your iPhone's Contacts list in addition to your Facebook friends.
Texts, chats, emails, and messages are consolidated in the same interface. Facebook hopes this will go a long way toward streamlining communication with your friends, particularly when trying to co-ordinate plans with large groups of people -- something that traditional email, SMS, and even the standard Facebook app's messaging interface all make much clumsier than necessary.
Facebook Messenger is not yet available on the App Store as of press time, but it should be showing up sometime soon. It will be a free download, but you'll obviously have to be a Facebook member to get full use out of the app.