It does suck when this happens, but there isn't much that can be done.

The longest I've had something run was about 5-6 months. After that I lost the people that had said they were going to assist me in managing the thread, which made it impossible for me to go on.

Sometimes though I've noticed that people sometimes just don't sync up stylistically which causes people to leave - as in the others they are writing with don't inspire them and posting becomes a chore. Just some thoughts.