Bill Ives wrote an interesting initial look at Booz Allen's Social Knowledge Management software. Booz was wrestling with the task of getting 18K employees worldwide to collaborate more effectively and intimately. They are solving it with a solution they call Hello.
My day job is working for a professional services firm. Although we are much smaller than Booz, we wrestle with the same basic fact: The bulk of our knowledge capital exists in the heads of our employees. Our most effective knowledge management system right now amounts to little more than an email distribution list. Questions are sent to the group and responses come in the form of a reply to the group (usually with an invitation to connect on the topic out of that loop). While this is an effective way to get answers quickly, the system rarely captures that answer. As a result, you see the same questions being asked more than once and often different people providing answers.
What Booz did was interesting on a couple of levels. First, they made people the molecule of the system (not documents or answers). The primary goal is to connect people to people in a tagged manner. As users connect themselves to others, they specify the nature of their relationship. The result is, essentially, a "tacit knowledge ontology" of the firm. An org chart based on knowledge and interests rather than corporate hierarchy or project data. The second thing they did was to tightly integrate the content generated by people with their profile. That way, a search uncovering some content can quickly be associated with a person and the user can pivot-browse the organization based on those ontological relationships between people to find the people with the answers.
It is also interesting that they did not mandate participation on the platform (either explicitly or by removing extant platforms). They let the value of the system be spread virally throughout the organization, and it's working. I've seen a number of efforts to get enterprise social off the ground and it's a difficult thing.
I think the thing people are most interested in is their relationships with others. This is why Facebook is so popular. Implementing a system that creates value through usage is the key to a successful social KM platform. Implementing a system that fosters interpersonal connections is the key to generating usage.