Re: Poor design
I equate FB chat with text messaging, but for a different group of friends. It's never been my experience that I or my friends camp out and wait to see who's on Facebook to send a message -- I never expect anyone to respond instantaneously (though sometimes that happens and we strike up a conversation).
I think everyone has their preferred mode of communication. For example, I have friends who won't respond to emails, but if you message them on FB, they'll reply. I have others who don't use FB chat, but they'll use Gchat. I'll email other friends to make plans instead of doing it via text. Depends on the person.