Facebook used to have the following URL format to compose a message:
http://www.facebook.com/?compose&id=XXXXXX&message=...
However, the above URL does not work any more: it is just forwarding user to Facebook home page. Does anybody if it is possible to compose a Facebook email via URL params?
This mobile link still works:
https://m.facebook.com/messages/compose?ids=USER_ID
As of May 2015, I have not found a desktop link that works.
Don't know if it helps, but I use:
It links to the Facebook Messenger.
You can use this link at this moment. User id can be numbers and actual name if the receiver sets it:
http://www.facebook.com/messages/[user id]?msg_prefill=message
It seems this old style works as well. It just redirects.
http://www.facebook.com/inbox/?compose&id=[user id]&message=foo
Update As of November 8, 2012, Facebook redesigned its Message UI and msg_prefill nor message parameter no longer works.
There are actually two options for a Facebook message link generation:
https://www.facebook.com/messages/t/[USER_ID]
or
https://m.me/[USER_ID]
If you want to be informed on changes, a bug was opened on Facebook on October 26, 2012.