I just found this neat little feature on Facebook which allows you to send prefilled messages to a person on Facebook.

Fill in:

id (must be the number id)
subject (string)
message (string)

The whole link looks like this:

http://www.facebook.com/inbox/?compose&id=652519918&message=I%20am%20writing%20to%20you..&subject=Just%20checked%20your%20blog!