Perhaps this is the big curse of acquaintances and friendships across the internet. Someone can all but vanish off the face of the earth, and it will be weeks if not months before his friends start wondering whatever happened to 'that guy'.

Oh, absolutely! I think it's due to how content is pushed to us: we don't have to poll someone and talk to them, we just see their updates on our friends page (or wherever) when they happen; when they cease updating, we'd have to switch mental models and starting polling them after all. But in the absence of updates from them, there's nothing to remind us, to trigger that switch.

I think that's one of the big downsides of this form of communication: people aren't really on our minds anymore to the extent they used to be.

BTW — totally unrelated, but I love that icon of yours. :)