Yammer vs. Email: what's the business case?

[Bjorkmeister]

OK, so I've dug around a bit trying to figure out a solid business case for using Yammer (or other social media tool) to make workers more productive. Most of what one finds out there relates to using social media to improve relationships with customers. I'm interested in making knowledge workers inside a very, very large company more productive.

So far I've found generalizations. Social media promotes serendipity. Great. How do I make money off of serendipity? Don't get me wrong, I intuitively see value there, but I need something more substantial than that.

How is it that the functionality you get from social media changes the game vs. email?
More details: my company is a multinational company in which collaboration among global teams is important.