What happens when people stop, die, or go away?
I'm a little worried. I used to follow someone on duolingo then he/she suddenly stopped all activity. One wonders why? Although the structure of Duolingo is such that we can't become personal friends there is still that feeling. I am sure that most of us feel close in some sense to the people with whom we have interacted on this site. Moreover, in the event of the worst happening, which it surely and inevitably will, what steps could or should Duolingo take to be sensitive to those affected by adversity?
This is an interesting question. I have no definite information about any Duolingo practices concerning such situations, but I think there are no special procedures. The accounts of people who die just remain as they are or, if the passed away person took care to leave his/her passwords to somebody, his/her account can be deactivated upon request.
There are some external services you may be interested in: http://www.thedigitalbeyond.com/online-services-list/. They can be used for passing the Duolingo password along with any other information.
Anyway, I'm sure a lot of people stop their activity just because they get bored. If they turned off their email notifications, they won't even know that somebody posts in their stream and wants to get in touch.