I've been part of a few web based communities and ended up over at Reddit about a year and a half ago.

I started out on the internet back in the ealry 90s where my interest in motorcycles drew me to AOL's motorcycle discussion group, then I discovered usenet discussion groups and eventually Yahoo groups. When it became apparent that the Yahoo sportbikes group wasn't being moderated (and it started getting spammed with ads) I moved to sportbikes.net where I had a home for almost a decade and eventually became a super-moderator.

I generated a lot of content for SBN. I wrote stories, posted articles, was active on the boards etc. One day, the guy who owned the site came on and announced that he had sold the site to a company called "Verticalscope." There weren't any real changes so I stayed on for a while, but eventually realized that I was being taken advantage of. I wasn't really a valued part of a community, I decided, I was working for nothing while someone else made a real profit from the content I was generating.

I think other members started to get the same feeling and the number of regulars on the board started to dwindle. Some of us eventually found our way to another site and started to make a go of it there, but then I was told that the guy who started this new site was none other than the guy who sold us out the first time. I was pissed, but I had friends there and stuck with it for a while - although I limited the amount of comments, articles and other content I generated.

Eventually I found Reddit and decided it was a decent alternative to my old haunts. There was a lot going on there and it seemed like fun. Of course, I missed the on-line relationships I was able to form when I was part of a smaller community, but the amount of content at Reddit was addictive.

After a while, I got more involved and started seeking out links and posting up self posts to add to the community (and I won't lie, build my karma). My posts were generally well received at first, but I have noticed over the past few months that just about everything I post now gets burried. Maybe I'm posting the wrong kind of stuff, I don't know, but I think the atmosphere there at Reddit turned unpleasant of late and, like so many others who have found their way here in the past few days, I have decided its time to make a change.

So, in answer to your question, I think it is a couple of things that make a site die. Casual users will quit coming to a site when they change their layout (for example, I never go to Jalopnik anymore because they changed their site layout about three years ago. I hated it so I left).

More important, I think the core members of a community opt out when they are made to feel unappreciated or are taken advantage of. When the core users opt out, your original content drops off and there is less interesting stuff to read. Without fresh, quality content people won't stay long and they won't get involved. When people hit your site and leave after a few minutes, you are done.