Facebook has been hiring some ex-apple employees to work on the new phone and if the phone is released it might even be a half decent. Hundreds of apps and websites currently give you the option to 'Log in with Facebook' so the Facebook app store will give you a good verity of games and useful apps.
The closest thing Facebook has to it's own mobile phone the HTC ChaCha. It has it's own Facebook button at the bottom of the keyboard which lets you share photo's and video's instantly.
The HTC ChaCha and Windows Phones have Facebook chat integrated so if a Facebook phone was released what new features would it bring to the smartphone market? Most mobile operating systems have been designed around social networks such as Facebook and rumours say that the next iPhone OS will be Facebook integrated.
So although the Facebook Phone might attract some dedicated Facebook-ers I doubt it will be a success because everything you can do on Facebook you can do on other phones plus more. And although the Facebook app store might be big it won't be able to match the application markets of Apple and Android. We still don't know the details of this phone or the price however I think unless it announces something revolutionary we will end up with just another company which has tried and failed in the smartphone market.