The official facebook messenger app comes to replace SMS via Facebook on-site and mobile notifications.
Facebook messenger is a brand new app from facebook for Android and iOS. This is not a facebook chat application but a messenger application for short messages. You can send messages to your facebook friends as well as non facebook contacts in your address book using it. It will create a facebook notification as well as a mobile notifications as standard text message for phones to ensure that the recipient gets the message one way or the other.
The app adds more features like location marking, photo attachments and group messaging which are nice addons. The messages are kept online linked to your account so you can visit the history both in the app as well as on Facebook website.
Get Facebook messenger @ iTunes / Android market