Yammer (www.yammer.com) is a leading provider of enterprise social networks with over 5 million verified corporate users including employees of more than 85 percent of the Fortune 500. The basic version of Yammer is free, and customers can pay to upgrade their network to receive advanced administrative and security controls, integrations with enterprise applications, priority customer service and a designated customer success manager.
We use yammer a lot as an internal discussion solution. Whilst within business units and projects/accounts Teams is used and is definitely more feature rich. For company wide discussions (250,000 staff) teams should be far too cumbersome to manage effectively.
For me Yammer is much more of a large scale enterprise tool.