Same experience here. Among my MSN friends it was also common to say "brb" to pause the conversation for a short amount of time. In contrast, I've never seen that in WhatsApp conversations.
Ah, yup! I had forgotten about that. Brb and afk. It did really feel like you were being ignored if the person wasn't responding. Also, having to let people know if you were going to be gone for an extended period of time, like a week or more. Summer camp! That was the worst. This girl I was online-dating went to camp for a few months and came back with a new boyfriend, lol.