I think the folks in Redmond, Washington are learning their lessons. They are letting Yammer remain independent like they did with Skype. This is an innovation move to keep the ideas and creativity flowing in different and smaller and more nimble workplaces. As long as they don't try to "over control" these newer acquisitions they might have a chance at keeping up with younger and hungrier companies.
The purchase also tells me that Microsoft understands the future promise of corporate social networking. It will be interesting when a large city or other government enterprise adapts the software to improve their own productivity. That can't be very long from now!