We do a LOT of communication via email. We often use our email as document storage systems ("Let's see... I know I sent that word doc to Bob back in May... of '06..."). With open copies sent on the cc line, email acts as a discussion board, helping bring people together to work out ideas.
Email is selective -- you decide who you're sharing information with, and it's one of the few Internet based systems that doesn't get spidered by Google. It still lets your ideas out "in the wild" if someone decides to forward your note, but it's like having a conversation in the office, not standing on the stage with a microphone.
At one time the Internet was filled with chat rooms which in turn evolved into threaded discussion groups. I'm not entirely sure why these seem to have fallen out of favor, but I still see them when I'm looking up technical answers.
The nice thing about threaded discussions is that you can come in late to the game and see replies directly below previous comments. Sometimes you get a back and forth going -- this conversation can wander off into its own corner and you can read the direct responses to the original comment.
I'd like to see Wiki's discussions become threaded, and maybe that's out there somewhere...
Teagan in my office pointed out that Blogs are linear; that is to say, blogs are a good place to record things as they happen. This goes well with my idea that blogs are like open journals -- be careful what you put in your journal if EVEYONE can read it.
I think of blogs as a great place to sum up ideas or things you're working on. Nothing is ever really a finished work, but the thing about blogs is that you can go back over time and look at the evolution of an idea and see how it's changed.
If blogs are linear and provide history, wiki's are "what's true now." I like the fact that wiki's have a history component, but the change history for a document is very different than a series of different documents on the same idea.
Wiki's are a great place for clearly thought out documentation that's constantly changing. We have our project management system hooked up to a wiki so projects and clients can have a back story -- who is this client, why are we doing this project and what are some of the things we need to know to stay out of trouble? It's a great way to keep documents in a semi-public place (that wiki isn't available outside the company).
We're also looking at setting up a wiki to let our experts within the company document important aspects of their area of expertise. For example, Kristen wants to be able to provide documentation about the importance of staffing and Human Resources, and this may tie into our accessibility division where they co-edit documents on accessibility and staffing.
But let's not forget good old fashioned web content. Your website will always have information that changes, but there are things that aren't open for everyone to edit and aren't always changing.
On the other extreme, your website should include "machine generated content" or data driven content. We still need a new word... there's all that data that gets processed, like order information and shipping status. This is content, and shouldn't be ignored, and in a sense, it closes the loop as it can generate new questions, new discussions, a blog or two, and update to the wiki, and a change to the company content.