Info from Major Nelson’s Blog :
A few moments ago we’ve released an update to the Twitter, Facebook and Last.FM applications on Xbox LIVE for all Xbox LIVE countries except for the US. With this update, as promised, if you have a child account and you get parental approval, you’ll be able to have access to these applications. At this point, all Non-US Accounts should have access to them at this time. If you have a child account in the U.S., you will receive this update on December 15th, when these applications will appear in your Xbox 360 dashboard. Be sure to set Automatically log in or Remember me when setting up the application for the first time, otherwise you will be prompted for parental permission every time you launch the application.
Thanks for your patience as we worked to develop the best solution to ensure that we are able to offer an age-appropriate environment for everyone on Xbox LIVE.