I have a message in Facebook that I want to send to another Facebook friend. How do I go about doing that?
If it is an extensive discussion and it cannot be taken off of Facebook (mail/skype etc) then use Groups as temporary message boards. The current flaw (last time I checked, this could have been removed by now) in groups allows you to add friends without having them accept. You can then start the message there and those who have been invited will receive a notification. Use with caution this can be used for malicious purposes.
Otherwise it is not possible without just copy/pasting the message.
You cannot forward Inbox messages at this time or loop someone into a message thread once it has begun.
Source - How can I forward a message or add someone to a message thread?
For more information please read up on Messages and Inbox : Sending a message in Facebook Help Section.