You might not have the update notification yet on your Marketplace tile but there should be one update waiting for your Facebook application. Just go into the Marketplace, search for Facebook, enter the app page, hit back and enter the page again to reveal the update button.
The new version is 18.104.22.168 and it appears to bring support for Windows Phone 7.5 dubbed Mango. You will now be able to pin elements to your Start menu as Tiles, such as your News Feed or Messages and Events. There are also added items for Toast and Tile notification for a more comprehensive list of events you are being notified of.