According to Facebook they released their new messaging system way back, http://developers.facebook.com/blog/post/591/
From the Blog:
Apps should move to the new messaging Graph API endpoints and messaging FQL tables ('unified_message' and 'unified_thread'). We will be formally deprecating the old messaging REST APIs, Graph APIs and messaging FQL tables at a date to be announced in the future.
But I still getting the "must be a developer". Is this just me, or does this work for anyone?
I'll try to use unified_thread table from FQL. Facebook suggests to move to that table, but it's still not working after 6 months?