Doesn't work AFAIK, though I believe there was a purported leak that offline messaging works with a certain version of Tango. I'm running 8773, so I'll try it out in a second.
I think where Facebook took its messaging runs counter to what Microsoft is doing. I used to use Facebook messaging like WLM, while my inbox was for more personal, thoughtful communication.
Facebook went and combined them, and it really annoys me. If Facebook kept them separate, I'd want a separate interface for the inbox. Now, threaded messaging is a convoluted mess if you want to check any history.