Andy Roberts links to a discussion by Miguel Cornejo Castro*, Blogs as community killers? Essentially, the question is whether blogs build or tear apart other online communities (listservs, online forums, etc.). I've seen this kind of discussion in several other places (mailing lists) as well. And I've talked about the idea of Blogging and communities previously.
The short version: It depends (I'm a consultant!). Sometimes blogs result in a community dying away. Sometimes they help strengthen a community. Sometimes they help build a community. Communities rely on many factors to grow and live.
There are some clear mechanisms by which blogs can lead to the death of communities. Primarily, blogs consume time and energy. If active community members shift to blogging, then they may not have as much to devote to the community. This is particularly true if their blogs focus on topics that closely resemble those of the community forum.
In a related sense, since blogs create their own communities, there is the potential to pull away the blog author AND her audience, and I suspect this is the real killer. If a community is lead by an active member who shifts their primary discussions to blogging, then I could see a large chunk of the community moving to monitoring and commenting (and blogging themselves) in the blog community, rather than the community forum.
As the blog community is essentially centered on individual bloggers, they must spend the time managing their own community: their blog, other blogs they read, comments, etc. More activity redirected from other community forums.
Blogs can also strengthen communities. Blogs are perfect places for longer articles and discussions. People with a bent for longer thought pieces now have a perfect place to post their articles for wider distribution. If they want discussion of those pieces, they can take them to the community and suggest a conversation. The benefit to the community is that the long posts can be diverted to only those people who are truly curious about the topic.
Similarly, blogs gives people the opportunity to post thoughts about their varied interests. As most people have interests that exceed the bounds of official communities, blogs are great places to expend that energy and leave the blogger energized for community-specific discussion at the community forum.
Active bloggers tend to read other bloggers and other sources of information that the official community may not consume. When items come up that are relevant to the community, the blogger can bring these new viewpoints and voices to the community for interest or discussion.
And blogging can build communities. People who didn't know they shared common interests can find one another. And if there is enough interest and energy, they can start or join communities that have to do with that shared interest.
One final thought on community longevity: community members get to know one another and want to hear what is happening in one another's lives. Blogs are a perfect place for details that simply do not belong on most community forums. Working in both spaces gives bloggers and community members the opportunity to focus when needed and still get a breadth of information about the people with whom they are interacting.
* Interestingly, I am familiar with both Andy and Miguel from various online communities. It's nice to discover Miguel's blog.