I think of this from both sides of the picture. SL means different things for different people, so the reasons they disappear can vary. I personally am an introvert who can be highly social, so it can be very confusing to people when I am online and we talk.. alot.. then suddenly I don't log on. And, frankly, my reasons for not coming on are varied: 1. I dont' feel like it 2. I am busy with my life, kids, home, husband, family, work.. take your pick. 3. I need a recharge, and I do that by being alone. I do not necessarily tell anyone these reasons, as I don't feel it is necessary. I never ask anyone why they haven't been on, I just assume they have a good reason and leave it at that. For my long time friends, they have picked up on my patterns, and some limited friends can contact me outside of SL, such as my RL FB or email.
I always let people do as they wish, and don't ever expect anything from anyone, even those who are in contact with me on a daily basis, therefore, if someone disappears for a bit or stops talking to me, it doesn't really bother me. Sadly, alot of people get in over their heads with a relationship, and then dont' know how to put the brakes on, so to speak, so the easiest way to do it for them is to simply not log on or ignore you. Basic communication skills and just being honest as to what they are experiencing would go a long way, but it is hard for some.