The Twitter and Facebook tabs can be found in the My Community channel on the Xbox live's dashboard.
"With the Facebook add-on, Xbox Live gamers can sync their gamertags with their social-networking accounts, as well as share photos and in-game screenshots. Friend updates will also be viewable through the Xbox Live dashboard"
When Microsoft added social networking functionality to all Gold-Level subscribers except those under the age of 18. Today Microsoft announced that those parental controls have gone in all other countries except the US. The US launch is scheduled for Dec 15th
The parental controls will now allow parents to approve children access between the ages 13 - 18
"The update went live today in the EU and everywhere except for the United States where it will go live on the 15th.
Minors must use the auto log in option for each app, or risk being prompted to ask for parental permission each time they want to use the apps."