Social networking for enterprises

Web 2.0 as they say is all about social networking, social networking and a stream of names flashes through my head ranging from orkut.com,facebook.com,friendster.com,linkedin.com etc

but often the trend previously was to block these sites in offices but recently there has been changes to these rules and there are people who speak in favour of these sites i am reffering to ceo's or cio's 

Now there are social networking sites who say its for the enterprises...

http://www.cubetree.com/