According to reports from Spanish website Mi Movil Windows (via WMPU), Windows Phone users will not be able to send messages to their Facebook friends even if the contact is offline.
Older version of the app only suggest to send SMS to the contact if it is not available. Now users can send offline messages. On the other end, user will receive the message when it will signin on Facebook.
It seems that Facebook and Microsoft has made some changes in the app and integrated the backend to support this new feature.
This new feature will surely make a lot of users happy. Nothing about these changes has been announced by any of the company. We are expecting some official release soon.