There are so many reasons why an online community might fail. Here's a starter list:
- No time investment from the founder(s)
- No compelling reason for people to coalesce (e.g., a community about Windex)
- Inconsistent or confusing administration/community guidelines
- Community founder(s) don't have an existing group to form the core of the membership
- Technology failure (need to use a platform that suits the audience, whether it's commented blogs, forums, chat rooms, or something home-grown)
- Too much "command and control" from the leadership; members don't feel an ownership stake
- No promotion; community is launched and then no-one knows about it
- Migration to a different community. Sometimes communities behave like flocks of birds...they decide to go somewhere else for any number of reasons.
- Founder(s) allow spammers or trolls to take control of the community; this will cause failure as the desireable members will flee.