Like the burden of proof, but for instant messaging. When two people (usually acquaintances and not close friends) are messaging each other, the burden of replying is on the
person who did not send the last reply. In other
words, that
person is the one who has to decide whether they want to continue the conversation or not, and if they do, they must be the one to send a reply or the conversation is over (unless the other person double texts).
Mary: When's the last time you talked to Joe?
Sue: Oh, I texted him this
morning but he stopped replying so I think he was busy or
something. I'm just going to wait until he texts
first; the burden of replying is on him, after all.
Mary: True.