These people seem to think saying something as simple as 'bye' or 'cya' before they leave your IM conversation is too difficult, so they go without saying goodbye.
Person #1: Morning, instant migrator
Person #2: What?
Person #1: You instant migrated from our convo last night
Person #2: So?
Person #1: You do it all the time.
Person #2: I CBA to say goodbye. (Person #1 walks away annoyed) Wait where are you going? Your just gonna leave without saying anything? I would never do that to you.