Twitter And Facebook Integration Missing In Notification center?

I have recently installed Mountain Lion and noticed that 'Sharing' via Facbook and Twitter are missing!

I have checked the System Preferences to enable this feature but I am stuck. Can anyone help me?

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 -> Mail, Contacts & Calendars -> Add Account -> Twitter (or Facebook). Once you've successfully added a Twitter account, the Twitter widget should appear at the top of Notification Center.

For twitter:

Go to your system preferences, type in twitter

you should get a drop down menu called Set up a Twitter account:

click on Set up a Twitter account and you should be taken to a screen where you can enter your twitter details

now for Facebook you'll have to wait to the fall of this year (2012) since apple will add this feature later. ( or if you have a OSX developer account you could get early access to the beta version )

note: you also can setup 'share' accounts for vimeo and flickr: