It's been rumored for a while now, but the deal is now official: Microsoft just announced it is buying enterprise-focused social networking provider Yammer for $1.2 billion. Yammer will join Microsoft's Office division, with the team continuing to report to its current CEO David Sacks. It sounds like the service will continue on in its current form, at least for the near-term — Microsoft says that Yammer will continue to be developed as a stand-alone product, with the company looking to accelerate Yammer's adoption alongside complementary offerings." At the time of purchase, Yammer had over five million corporate users, with employees 85 percent of Fortune 500 companies using the service. Of course, this deal is subject to regulatory approval and other closing conditions, so we'll be looking to see if it hits any roadblocks along the way to passing — Microsoft didn't give a timeline for when it expects the deal to be complete.

Update: Microsoft just held a conference call with CEO Steve Ballmer, Microsoft Office Division President Kurt DelBene, and Yammer CEO David Sacks. There weren't any earth-shattering revelations, but everyone reiterated how much they're looking forward to integrating Yammer into other Microsoft products, while also reinforcing the value of the standalone Yammer software. Sacks again confirmed that the Yammer team would operate as it does now and continue to report to him; the team will remain in San Francisco, and Sacks will report up to DelBene. When asked whether the Skype model was a good one to look to for how Yammer would be run, Ballmer responded "sure," but also said to think of Yammer "as a fundamental part of our Office family" — it's pretty clear that this product will be making its way into future Microsoft releases even while existing as an independent piece of software.