Which makes Facebook's case interesting, since you can't control through which UI your recipient will view your message.

I once had to reconnect with someone whom I only know how to contact via Facebook, and sent an email-style message broken into a few short paragraphs. I did this in the "full screen" messages window, and later happened to view my sent message in the tiny chat box.

If I had received that huge message in a tiny chat box I would have had a very negative reaction: "Ugh, what does THIS guy want and what's he rambling about?"