Introduction
Starting with OS X 10.8.2 and future versions of OS X, Facebook will be integrated directly into Mac OS X. Users will be able to sign into Facebook via their System Preferences, just like they do with Twitter now in Mountain Lion. Once signed in, several new abilities are unleashed.
Apps such as Safari, Notification Center, Game Center, Preview and Contacts will now include a “Facebook” option in their sharing.
To Enable Facebook in OS X Mountain Lion10.8.2, follow the below steps:
1. Open System Preferences. Do this by clicking on the Apple logo 
in the top left side of the screen and then click on System Preferences…
2. In System Preferences and under the Internet and Wireless section, click on Mail, Contacts & Calendars
3. Inside the Mail, Contacts & Calendars preferences, click on facebook, type your login information for facebook, then click Next
4. The next page will describe to you the benefits of integrating facebook with OS X Mountain lion. Click Sign In
5. You will be signed into facebook and your username will appear on the left pane.
6. So now that you signed in from Max OS X into facebook, you can start posting to your facebook account directly from OS X itself.
From the Notification Center
You can type directly to facebook by clicking click on Post
From Safari :
If you want to share any website, then click on the share button beside the address bar, and then click Facebook
Using Quick Preview
Quickly preview any file by selecting it and then clicking on the Space bar. When the file is opened, you can click on the share button
Summary
As it happened before with iOS, where Twitter was first intergated with iOS in version 5.0 and later on Facebook was Integrated with iOS Version 6.0 , the same happened with OS X Mountain Lion, where first Twitter was integrated by default with 10.8, and later on with OS X Mountain Lion version 10.8.2, Facebook became integrated. Apps such as Safari, Notification Center, Game Center, Preview and Contacts will now include a “Facebook” option in their sharing.