Current online systems are good at showing you when someone does something, posts or tweets or toots or comments or whatever. But the endless scroll hides the absences. Not out of any malice, I suspect, just because it's harder to show people when someone doesn't say something. What would that notification even look like?
Nate, your thought reminds me how much software focuses on entrances. “This is the first time Nate has posted – let's welcome him to the community!” People join and introduce themselves. All is well and good.
Then people slip out of the conversation. These are quiet and drawn out. No formal goodbyes. No “I miss you, please come back.” It just happens.
There are people I follow on the Read Write.as feed, for instance, that seem to have stopped writing. I note how I have not seen their posts in a week. Then a week becomes a month. Then I forget that I miss them. Not out of any malice, I suspect, just because it's harder to notice when people are not posting when they used to.
While a community based question, it also sounds like a systemic one. Why can't we build into our software the ability to reach out to lapsing members, to remind us of people who we should reach out to again, to let someone know that people are missing their contributions on a platform?