Great question. From our user research we find that when users are in Yammer, they may see something interesting, and may quickly share it with someone else - right in Yammer. These conversations may eventually be shared in a Group, becoming useful to more people in the organization. But if they are reading a thread, and want to instantly call a person from right inside Yammer - we think Skype would be a natural fit for such audio/video calling scenarios. If users want to start a private message - when they are not in Yammer - they typically use Email (for long form messages) or Skype (for instant messages).