Wow, Tinkerbell and JeniBean, you guys are really getting at my heart this morning! This is SOOO much the stuff I've had to deal with, especially in the last few years, and -especially- with internet friends, who seem particularly fond of leaving without saying goodbye. I think the internet environment just makes it really easy for people to do that, sadly. My real-life situations with this tend to involve extended family members who have dysfunctional ways of interacting with everyone, and I just end up getting the very painful end of that bargain.
I have experienced different kinds of goodbyes...the articulate goodbye, the sudden goodbye, and the mysteriously silent goodbye. By far, the worst of these to get over is the person who just leaves. It creates worry. Not just this, but the person who just leaves frequently decides not to communicate about it at all, creating a "shut you out" effect, leaving you wondering, "What did I do?" to create the situation, or to make that person leave in the first place.
I don't know if the other party realizes how painful this is. I don't know if they're just lazy or avoidant, or what the issue is. I do know that it is a horrible way for people to handle things.
You should -always- say goodbye. No closure is the worst.