Prairie South School Division in Saskatchewan uses Microsoft Outlook for primary communication. After many years it seems to have caught on. We also have a forum for professional development that is poorly subscribed to. People don't seem to understand how forum thread changes can be alerted through email. Our division also has a private Yammer. This is is only used by a handful of staff, although a good number have joined. I think the problem with the forum and Yammer is that those of us who do use social networking applications for professional growth have already tapped into wider networks. There is only so much time to participate and share.
Where possible, I use applications like Twitter (stangea), Buzz, Google Reader, and Posterous where posts and comments are linked together or distributed widely. The best value is writing a comment one place and having it distributed to a number of sites. My email gathers in the forum discussions and posts to my Posterous are distributed to other sites. I also embed feeds (like Twitter) into other pages.
Outlook is an old application but I think it is excellent.