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Twitter for "Mountain Lion" takes a leaf out of iOS 5's book. You can also update your contacts with your Twitter contacts so you can contact them from your iPhone or iPad.
Safari has a few new features. One of them allows you to tweet your friends with articles and other things. This is one good example of using the "share" button (located next to the back/forward buttons) to share content with your Facebook friends or Twitter contacts.
The same works with Facebook. You can post website links or images to your Facebook timeline, and even edit who gets to see it -- including preventing certain lists of Facebook friends. You can even add your location to your statuses.