Getting Started with Messages
How do you get started with Messages? It’s easy, once Facebook invites you. (The invitations are slowly rolling out across all users.)
Follow the instructions on the invitation screen to join the new service, then provide the information requested. You’ll be asked to choose an email address (typically your Facebook username) and provide your mobile phone number (for receiving text messages), and you’re set.
If, perchance, you opt not to provide certain information at the start, you can still add that info later. As long as your information is incomplete, you’ll see a small pane at the top of the Messages pane; click any link here to fill in the missing info.