Simple: because there is no open protocol yet to exchange data between these system. It's called a portable social network, and we need to decide together how it will shape the future of online networks forever.
Facebook and Twitter are leading the way showing how social media can be open. OpenID is a burgeoning standard that can be used for single sign-on as well as moving user’s data between systems. Microformats are paving the way for a structured web. Come meet some of the movers and shakers helping to create portable social networking.