I found an excellent bit of text from an article on NewsForge that clearly outlines when it is typically best to use either a wiki, a blog or e-mail to communicate. Here it is:
Before you start, consider blogging’s place in your workgroup’s communication infrastructure. A wiki is an outstanding tool for public collaboration, while email excels at person-to-person communication. Blogging occupies a useful niche between these.
Blogs can provide an important venue for certain kinds of information. For example, a business development manager can provide daily reports on sales calls without waiting for some specific progress to report (e.g. a contract signing) as he might want to do before sending a company-wide email. As a result of having more information about the manager’s customer visits, participants in design meetings may be better equipped to balance upcoming feature releases.
As another example, a project architect could release a series of blog entries detailing long-term goals and intentions for a project. Co-workers could then respond to, and help refine, the architect’s ideas. At the same time, the employees taking direction from the project architect will develop a better idea of what the architect has in mind and pursue their own work accordingly.
In short, by providing an informal window into a person’s or team’s work, you can foster better communication between co-workers.