If you've upgraded to OS X Mavericks, you may have noticed that Safari has been updated too. The Shared Links feature in Safari can pull from Twitter and LInkedIn so you can easily access all the links your friends are sharing without ever even visiting each respective website. Here's how:
If you don't see any links populating, hop over to System Preferences and under Internet Accounts and make sure you've got your social accounts linked to OS X.
That's all there is to it. No need to actually use Twitter or LinkedIn in order to view what your friends are sharing while you're using Safari.