Yammer almost always gets its start in organizations as a grassroots initiative where "virality lets us start the conversation," but in success stories like the Supervalu grocery chain, that meets a top-down recognition of the value of the social software employees have already started using.

Meanwhile, Sacks is as dismissive of competitors like Jive and Salesforce.com's Chatter as they are of Yammer.

Like a lot of software vendors, Jive likes to associate itself with the idea of "the cloud" but spends a lot of time talking about "private cloud" and "hybrid cloud" scenarios that were "invented by people who didn't really believe in the cloud," Sacks said. The cloud is just another word for the Internet, and true cloud computing services live on the Internet, he argued.

Sacks said Jive's Project Thunder, an upcoming product enhancement for self-provisioning that Zingale discussed during a recent earnings call, sounds a lot like Jive trying to adopt its own version of the Yammer cloud and freemium model. (Zingale said it would make it easier to offer free trials, not necessarily accounts that are free indefinitely).

Currently, Jive provides application hosting, but it doesn't operate in the same Software-as-a-Service mode as Yammer, where all customers are automatically upgraded to the latest software release. Many of those "public cloud" Jive customers are running a 4.x version of the software, months after the release of Jive 5. On the other hand, many of those customers like having the option of choosing when to upgrade--an option they wouldn't get with Yammer. Yammer's enterprise customers have some checkbox administrator options of what features to enable, but they get upgraded to the new version at the same time as everyone else, Sacks said.

That deployment model allows Yammer to keep its costs and prices low, while also forcing the service to be disciplined about preparing customers for coming changes and testing them to ensure a smooth rollout, Sacks said. Upgrades are incremental rather than sudden, so users don't have to adjust to too many changes at once, he said.

Salesforce.com is serious about the cloud, but "their idea of a social business is an organization that buys all their products," Sacks said. While Salesforce may be trying to convince people not to use Yammer and sign up for Chatter instead, Sacks insists the opposite is not true. "We're fine with people using Chatter. To us, it's just a feed for activity in Salesforce," he said. "Creating a workflow app for sales is a valuable application, but it's just not company-wide."

As an enterprise social network for the whole organization, Chatter falls short because it is too full of robotic activity stream updates from the Salesforce application, Sacks said. "The velocity is high, and it tends to push human conversation off the page." Yammer has taken a different approach, displaying automated updates in an activity stream that is separate from the main feed of posts from other users, he said.

Yammer is also actively seeking to expand the application feeds it has access to and just announced new technical partnerships, including a connector for SAP systems created by the consultancy Freeborders, and integrations with PlanView, Kindling, Sparqlight, and GageIn. In addition, Yammer added an affiliate program for consultants and integrators.

Yammer's approach is as much about consumerization as it is about cloud computing, Sacks said. He believes consumerization, defined as "enabling end users to have more control over the products they use," is yet another paradigm shift in business computing, as significant as going from the mainframe to client-server, or from client-server to the web and the cloud.

IT organizations that fight consumerization and the cloud are fighting against their own interests, Sacks said. The mindset that IT needs to control the pace of upgrades is a relic of another era when "the onus was on you to make sure it didn't break," but now cloud services can take over the drudgery of systems maintenance, he said. Meanwhile, accepting the consumerization trend exemplified by a freemium service like Yammer means "you don't have to be a cheerleader for adoption anymore," he said-- you can let employees use applications they like, rather than compelling them to use ones they don't.

Follow David F. Carr on Twitter @davidfcarr. The BrainYard is @thebyard

The Enterprise Connect conference program covers the full range of platforms, services, and applications that comprise modern communications and collaboration systems. It happens March 26-29 in Orlando, Fla. Find out more.