Microsoft reportedly has agreed to acquire enterprise-focused social network Yammer for more than $1 billion.

The agreement was first reported by The Wall Street Journal, and by other media outlets over the weekend. Microsoft has not yet made a formal announcement though the company has a press brief scheduled on an unspecified topic for later today.

If the deal happens, it could represent a key turning point in the evolution of the social enterprise movement. Along with Salesforce.com's Chatter, Yammer is part of a rapidly evolving effort to bring more social collaboration and chat tools into the corporate enterprise to spur communication and productivity between corporate teams.

If Microsoft does acquire Yammer, it probably could be integrated in some way with other Microsoft collaboration tools, such as the SharePoint platform. Social enterprise platforms also have been employed to improve customer relationship management activities, another area in which Microsoft could match Yammer with assets it already has.

Yammer has been particularly aggressive on the mobile side. Companies behind new mobile applications are often forced to put their limited time and energy into adapting their solution to one particular platform, like the Apple iPhone, but Yammer was available on the iPhone, Android, BlackBerry and Windows Mobile devices as early as 2010. Yammer actually was named one of the top applications of 2010 by FierceMobileContent.

Though Salesforce.com has been widely credited with leading the charge on social enterprise, Yammer also was one of the earliest such applications. The company also has been well-funded, and past reports have suggested that Yammer could eventually pursue an initial public stock offering.

Huddle is another company in the same space that recently raised new funding and hinted at an eventual IPO, perhaps showing that social enterprise companies have not been spooked by Facebook's troubled IPO, and also suggesting to larger companies that if you want to acquire a social enterprise developer, now might be the time.

Other firms pursuing social enterprise developments include Social Text, Traction and Jive Software, though the list of start-ups keeps growing.