In what could be Facebook trying to leverage the most ad dollars it could possibly get out it's mobile users they sent a letter to Microsoft demanding that apps that "infringe Facebook's trademark" be removed from the Windows Phone store. This means that most 3rd party apps that make your Facebook experience better on your Windows Phone device will be disappearing very shortly.
Full List of affected apps
If you wanted to download a new Facebook app I suggest you do it soon before it's no longer available to download. This seems like a bad PR move for Facebook sadly I don't think any real harm will come to them from it.