Question about facebook messenger etiquette?

[Boxjuice^2]

If someone starts a converstation with you, at a time when you cant talk (for example, when youre about to leave the house or go to sleep) are you supposed to reply telling the other person that, or do you just go offline?

[Jonathan]

I think it's nicer and more polite to simply tell them that you can't talk at the moment, and that you'll get back to them as soon as you can. If you simply appear offline, they may be sitting there awaiting a message for quite some time.
So long as the message is worded politely and respectfully, I don't see any reason for them to take offence or feel insulted.