Well, you can’t. Not just yet anyway but you can request an early invite to the service from Facebook.
Only a day after the announcement of Zuckerberg’s “GMail Killer”, an early invite system has gone live. Users of the social networking site who want to access ‘Messages sooner’ can now request an invite to the service over on the Facebook help site.
On the site Facebook says,
We’re rolling out the new Messages to everyone gradually, so you should see steps to upgrade soon. If you want to access the new Messages sooner, visit the Learn more page to request an invitation (invitations will be sent pending availability).
Facebook’s new messaging system is designed to aggregate emails, status updates and sms messages into one thread. Like GMail Facebook Messages will boast a priority-enabled inbox which will filter conversations according to sender rather than date. Read our article here.
As invites aren’t sent immediately you will have to wait for a bit but, you never know, you might get an early Christmas present.