Do you notice when fellow flutherites suddenly seem to be missing or have gone quiet?

If you do notice, what is your reaction? Send them PM’s, Tweet them (if you happen to know their twitter), worry about it for a hot second then move on? Just assume that they have gone on with their lives?
I have noticed a number of flutherites I used to see regularly aren’t really in the threads. I check their profile to see when they last log in, it’s just calming to see they are logging in even if they aren’t speaking. So…do you notice? If you do, what do you do?