We all have the desire to analyze the things in our life (groups of friends, clubs, companies, communities) that have died. I do it all the time. I wonder what happened to community "x" that I was a part of and really enjoyed and miss. You look around at your neighborhood or city and see permanence and figure your community must have had something wrong with it.

Of course, the barriers to entry and exit for an online community are very low. People change, the time they have available changes, their interests change, whatever. They exit the community as easily as they entered it. Soon the community dies. Your town or neighborhood isn't so easily left or entered so the stakes are much higher.

Sometimes communities just get old and people leave. Pretty soon they're gone.