This all being said, there are three occasions I can think of where Facebook messages are considered acceptable:
1) The quick dashed off observation or instruction (almost always Facebook-related) to a BFF ("check kim's new album now. lol. 36 and 40 especially.").
2) The "hey, check it out, this is a casual but flirty message" to someone you met at a bar or whom you notice has just updated his status to "back in nyc!" or whatever. These are the kinds of messages that cause you to squirm and shudder if you come across them months later (especially the ones that never got a response), but which - thankfully! - are pretty easy to block out completely once they've been sent.
3) The brief, friendly message to a potential new friend whose e-mail address you can't find/don't know. These are OK, sort of, as long as they are of the "lunch next wednesday?"/"here's my phone number" variety, and not the "blah blah blah"/"how are things at work going?" variety.