There is a difference between Microsoft creating a custom application for Twitter and Facebook, and Sony having a second rate web browser that can access them. It's the same reason as to why even though the new phones have web browsers, they still create custom apps to access certain websites. They function better that way, because they are functioning for the device in hand, not merely emulating the experience another device provides better.
To blow your argument apart, if the web browser does Facebook so well, then why did John Koller of Sony say in early April that they were looking to integrate the social networking sites into the PS3? In fact it sounds like they have the exact same plans that Microsoft just announced.