Anyone can sign up for a free Yammer account, with nothing more than a verified email address at a particular domain. Yammer groups all of the free accounts that share an Internet domain into a private social network--a great collaboration option for lots of small businesses and some larger ones. The company behind the domain only has to sign up as a paying customer if it wants to assert administrative control over that collaboration space. That's how it worked when Supervalu adopted Yammer as the grocery chain's enterprise social network. A group of employees started using the tool on an ad hoc basis, the company's CEO found out about it and saw the possibilities, and the unofficial collaboration environment became the official internal social network.
On the other hand, some CIOs and other company leaders see this as a problem. They find out after the fact that employees are collaborating and discussing all sorts of company business on an unsanctioned cloud service. When they learn the only way they can get administrative control over this environment is to sign up for a commercial account, they are pissed.
Suppose I don't want a commercial account, I just want to shut this thing down? Yammer's official response is that I can't do that--as far as they are concerned, these are a bunch of individual accounts that just happen to share a common email domain.
I know this because I discovered that there is a Techweb Yammer group associated with my email address (email@example.com), populated by other employees of the Techweb division of our company who at some point were curious enough to sign up for an account. It looks semi-official, but it's a rogue social network with no official standing. I also used this example as part of my presentation for a BYTE webinar on "The Rise of Social Networks in the Enterprise".
Our parent company UBM has an corporate social network based on Jive, but as far as I can tell has made no particular effort to shut down the Yammer alternative. Nor do they need to. Because all of our employees and all of the most active discussions are on the official collaboration environment, this Yammer instance is a sleepy backwater, mostly forgotten even by the people who do have accounts.