As Apple promised at the launch time of OSX 10.8 on website about Facebook support, now osx support Facebook fully integrated with osx with osx Facebook integration is now only available in 10.8.2 dev versions build 12C50 but it will be available for all after final release of 10.8.2 update
for integrating facebook with osx mountain lion go to system preferences and mail contacts & calendars menu and click on facebook
Its best practice to use facebook app specific password for these kind of services
facebook integration will import your facebook contacts and give you option to share pictures and update your statuts and many more functionality
Enjoy facebook with mac osx mountain lion