Yammer may still be known as the startup that offers “Twitter for the enterprise,” but it continues adding features that differentiate it from Twitter and make it a more powerful business tool. Today, it announced Yammer Communities, a way to use microblogging to communicate outside your company.

Until now, Yammer customers used the product for internal communication, which was enforced by the fact that anyone in a Yammer network needed to have a company email address. But chief executive David Sacks said microblogging can be useful for working with partners, customers, and others, and also to create new kinds of networks. He painted a picture of how a series of connected Yammer Communities can start adding up to “the B2B social graph.”

The San Francisco startup has already recruited a number of launch partners, and they give us a sense of how companies could use Yammer Communities. Deloitte, for example, will use Communities to discuss projects with its consulting clients. Cleantech company Better Place will use the feature to coordinate with its manufacturing and vendor partners. And an unidentified media company has built a community for an advisory panel of 1,500 moms.

Sacks also gave reporters a quick tour of the product, during which he emphasized the separateness of each community. Not only is each community a separate stream of posts, but members even have different profiles for each one. Members can bring up a list of all their communities, which shows the number of unread messages in each network. That’s an important step in making social networking tools a valid replacement for email, Sacks said, because that makes it easier to keep up with all of your communication and not let messages slip through the cracks.

Yammer plans to launch the Communities feature on March 1, on its web, mobile, and desktop interfaces, and the feature will be included in Yammer’s standard pricing.

During the briefing, Sacks also shared that Yammer has 60,000 customers and made “seven figures” in revenue during its first year. Yammer has raised $15 million in funding.