After trying the free 'Chat for Facebook' from App World I was greatly disappointed. It was not user-friendly and cumbersome to navigate.
The FaceChat App by JaredCo makes things a little easier. As you can see above, the chat interface is clean and familiar and you can use enter to send messages. The one hiccup people might run into is logging in. It requires your username which is different than your email you normally use to login to the site. You can easily find your username by going to your profile on a computer. Your username will be whatever is after the '/' after www.facebook.com in the address bar. For example: My username is jay.harker and I found that in the address bar of my browser. www.facebook.com/jay.harker.
Download FaceChat here.
Either that, or don't.