Facebook and Twitter on Xbox 360

Silver members on XBL, good news! Kinda. Facebook and Twitter will be accessible to you all. The 'kinda' part in this news: it's only for a limited time only.
Microsoft told VG247:

Facebook and Twitter on Xbox LIVE will be free to Gold members, with a free trial period also available for Silver members (emphasis ours). Facebook and Twitter on Xbox LIVE will offer a truly one-of-a-kind social experience, completely optimized for the TV and seamlessly integrated onto your console. Add to that exclusive access to Last.fm, Netflix instant streaming, demos, trailers and discounts, plus the best multiplayer gaming network on the planet, and we re confident Xbox LIVE Gold memberships is hands-down the best value in home entertainment. We re committed to adding new features and value for our Gold members, and will continue to do so.

There you go.