Hmmm... I have been playing around with some Facebook plugins here...
  • a Facebook Login button
  • a comment field which directly send the comment and a link to your profile
  • and the ubiquous 'Like' button

Twitter has also found its way here too... with the follow me link  
and the tweet buttons!

Google's Buzz is here and you can find it at the top of this page... look for the following icon: