So far Microsoft has largely stayed out of the social networking arena, choosing instead to partner with Facebook. Apple may be following this path, after Ping, their music-driven social network failed to catch on. Google of course is still in the thick of competing in the social space with Google+, and has recently brought a stronger emphasis on social networking from mobile devices.
Yammer allows companies to create social networks for employees to participate on, with the idea of increasing employee collaboration and engagement on work projects. Yammer has a strong set of mobile tools, which currently cover all the major mobile operating systems, including iOS, Android, BlackBerry OS, and Windows Phone.
Whether cross-platform support would continue is not known (although these buyouts usually don’t lead to immediate changes in platform support), but Microsoft would almost certainly look to integrate Yammer’s software into its Windows 8/RT/WP lineup (possibly make them a part of their productivity suites). We may find out sooner rather than later, as the infamous “people familiar with the matter” indicate that a deal could be struck as soon as tomorrow.