Facebook already integrates with Microsoft products: Bing is a search option on their site, and all of their embedded maps (Events, etc) use Bing Maps.

On the MSFT side, Windows Phone has always included deep integration with Facebook since the start, and their most recent acquisition, Skype, is the provider of video chat for Facebook's in-browser client.

Both companies already integrate with each other in multiple areas, and Facebook employees almost certainly aren't discouraged from using WP7/8 on their personal devices. The market realities (low market share, existing integration in the OS with no standalone, third party app to support) means that there's no need to push this on employees over iOS/Android.

I think things for the most part just don't work this way in larger companies.

Facebook needs to focus on what is best for their business. Pandering to investors could interfere with the long-term success of the business.

Also, the Windows Facebook app and integration is already pretty awesome.

Facebook will of course integrate Microsoft products with Facebook and vice versa. But that shouldn't stop them from offering Facebook to other platforms. And to do that well, they need to use and work with those other platforms. It wouldn't be a good idea for Microsoft to interfere in that department.