A hidden feature residing in Mac OS X Mountain Lion is the ability to view and also reply to your friend's tweets through the Contacts application, previously known as Address Book.
The only way to do this, though, is to provide your Twitter account login data to the Contacts app. To integrate your Twitter, just go to System Preferences, then click on Mail, Contacts & Calendars and choose Twitter. Put in your Twitter login information and then click on Update Contacts. Twitter will use your email addresses and phone numbers to add Twitter handles and photos to your contacts.
From there, you can look through your contacts for anyone else that has their Twitter integrated. You can use the keyword "Twitter" to help you narrow it down. The video below details the process.
Facebook is also integrated into the new OS X, as well as all the popular email servers. People may prefer going on the actual site to access Twitter, but this can be useful when in the office. It's quick and easy and shows the depth of Mountain Lion.
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