I ran a very large forum for a long time. The users there were all pretty intertwined in a lot of ways, they knew each other. It was a car community so there were a lot of close relations in the club. It was really weird because as the site owner I was often alerted to the death of the users.
It's just kind of a weird thing to have an active user, and then the next day they are dead. We also had a few deaths resulting from the same stupid negligence like not properly securing a car while it was up. That didn't happen often but it happened more than a few times over the course of 10-15 years.
Some people die, some move on... I always wondered what happened to some and I would often try contacting them as I was the only one able having their personal information and often time got no response back from them.
I actually wrote an article about this a bit back pertaining to our social connections today vs 20 years ago and how we experience death now in a different way, and how we really aren't prepared for it. I went into a lot more detail with the article, however to summarize it touched on Suicide and possible large spikes in Suicide because of these new connections and our lack of experience in dealing with that sort of loss on a mass scale, or humans putting less value on others lives as a coping mechanism.
Anyhow to answer your question OP, yes, I often wonder about people.
On the flip side, I've disappeared many times myself from various forums, chat programs etc where I knew hundreds of people. I logged back into my ICQ account from 15 years ago and had fun going through the old names of people I used to talk to daily.