I think people disappear for all sorts of reasons. Some go back out and use, some get busy with something in their lives. Some are off licking their wounds over some real or imagined hurt they experienced here or in some other aspect of their recovery support system.
It's hard to not get personally invested in a friend, cyber or not.
Forums can be drama magnets, and this one is pretty good about being stable, but one thing that tends to stir up a huge drama pot is someone posting that they won't be around. Some people do it specifically to get the strokes of people posting hoards of "please don't go" pleas. Or accusing one another of driving so and so away.
For folks who just needs some time off, they often DON'T bring it up, because they don't want to have to deal with all the "please don't gos, I need you man" pleas. Because part of them feels a little guilty about leaving good people they respect behind, people who've taken time to help them.
But yeah, it's hard. New hellos, real friends and goodbyes are part of the process of recovery and of life. We are learning life skills here, I try to take the lesson, even as I mourn the good bye. I've disappeared a few times myself, for all sorts of reasons, sometimes simply losing my net access. What's great is that every time I come back, it still feels like home.