This update is recommended for all OS X Mountain Lion users, and includes new features and fixes:

Facebook

* Single sign on for Facebook

* Adds Facebook as an option when sharing links and photos

* See Facebook friends' contact information and profile pictures in Contacts

* Facebook notifications now appear in Notification Center

Game Center

* Share scores to Facebook, Twitter, Mail, or Messages

* Facebook friends are included in Game Center friend recommendations

* Added Facebook "Like" button for games

* Challenge friends to beat your score or achievement

Other new features

* Adds Power Nap support for MacBook Air (Late 2010)

* iMessages sent to your phone number now appear in Messages on your Mac

* You can now add passes to Passbook (on your iPhone or iPod touch) from Safari and Mail on your Mac

* FaceTime can now receive calls sent to your phone number

* New shared Reminders lists

* New sort options allow you to sort notes by title, the date you edited them, and when you created them

* Dictation now supports additional languages: Mandarin, Cantonese, Spanish, Korean, Canadian English, Canadian French, and Italian

* Dictionary app now includes a French definition dictionary

* Sina Weibo profile photos can now be added to Contacts