Because Windows Phone 7.5 is integrated with Facebook, you could get away with not having the Facebook app installed, but a new version of it adds some features that I think will be really attractive to heavy Facebook users.
The new version of the Facebook app supports features that came with the Mango update to Windows Phone. You can now pin the Messages, Places, and Events sections of the Facebook app to the start menu. To pin these items you need to open the section in the Facebook app, then tap the pin button at the bottom of the screen.
When you enter pinned sections you are restrict to that section. For example, tap the Messages live tile and you will be able to access is the Messages inbox, updates, and sent. You cannot go directly from the Messages live tile to the news feed, to access your news feed you will need to open the full Facebook app.
The new version also provides more controls for push notifications. Go to the settings portion of the Facebook app to specify whether you don’t want to receive push notifications, see toasts and tile counts, or just tile counts. You can enable notifications for just about every major area of Facebook, from messages and wall posts, to photo comment replies and friend requests.
You will find the updated version of Facebook in the Windows Phone Marketplace.