Twitter And Facebook Integration Missing In Notification center?
The Facebook integration wasn't available when this question was asked - it was added in OS X 10.8.2 in September 2012.

To activate Twitter and Facebook sharing, you must first configure at least one Twitter and / or Facebook account via System Preferences -&gt; Mail, Contacts &amp; Calendars -&gt; Add Account -&gt; Twitter (or Facebook). Once you've successfully added a Twitter account, the Twitter widget should appear at the top of Notification Center.