If you've not seen this, there's a Facebook application by Worklight called Workbook that represents an overlay to turn Facebook into an Enterprise Social Network application. This is something I've been expecting for a while based on some conversations with large organizations who planned to use Facebook as their social network.
WorkBook combines all the capabilities of Facebook with all the controls of a corporate environment, including integration with existing enterprise security services and information sources. With WorkBook, employees can find and stay in touch with corporate colleagues, publish company-related news, create bookmarks to enterprise application data and securely share the bookmarks with authorized colleagues, update on status change and get general company news. Employees can freely use Facebook, with the WorkBook overlay, with no danger of information leaking outside the organization or access being granted to unauthorized personnel.