Say you walk up to two people having a conversation. It is obviously rude to just interrupt that conversation with whatever it is you want to talk about, and the norm is to hover about until you are acknowledged, politely apologize for interrupting and then say what is on your mind. Similarly, if you walk up to someone in their office who is on the phone, the norm is to wait quietly until either the person is off the phone or makes clear they can talk while on hold.
Yet, if you walk up to someone in a conversation with another person chatting on IM, the norm, as far as I can tell, is to assume they are doing nothing important, that there is no third person involved, start talking away right away. As far as I can tell, it is considered rude to ask someone to wait while you finish an IM conversation. And yet, that is a real conversation with a real person. That conversation started first and should take precedence. But somehow, because we are used to interrupting people who are typing, we haven’t caught up to counting this as a conversation in our etiquette.
I was the interrupted one today, and I couldn’t bring myself to tell the in-person conversation to wait, even though I wanted to, and even though I thought it was the right thing to do. Maybe the next generation will have a different set of internalized norms, but I am hopeless.