I'm not sure about a GPS navigation application but I believe that Zune, IE, imbedded radio have the music and any Facebook/WWW multitasking issues covered - because they are all 'native' applications. In fact, I've read you can even see some FB status updates on the main screen tiles if you want.
Not sure how it works with one 3rd party app running while other native apps are running in the background.
The Microsoft apps can still run, but as soon as you switch to one of them you lose your third-party app running. Basically, the problem is that lack of multitasking was a big complaint against Apple, so Apple introduced managed multitasking to allow it while saving batteries. Along comes Microsoft's latest and greatest phone OS, and it doesn't even do that.
It doesn't even do copy/paste, also an initial iPhone complaint, long-since rectified. It also doesn't do IPsec VPN. And for people who favor Android, WP7 also won't do Flash or allow removable SD cards. It's deficient in some way when compared to each of the main competitors. Right now, I can't see a reason to buy it, unless you just like the way it does the social networking thing.
In fact, I've read you can even see some FB status updates on the main screen tiles if you want.
There's an FB widget for Android too, and a Twitter one.