Twitter and Facebook, on your Xbox. It's weird, like people who put ketchup on their eggs.
Twitter actually makes the most natural jump to the Xbox. It's a really basic app, with your timeline, search, and trending topics, but it works, largely because the vertical stream is preserved, even if you can only see four (very legible) tweets at a time, so you won't be power-browsing, TweetDeck style, by any means. It's slow, and typing's reeeeeeeally frustrating, like having your eyeballs poked out one pinprick at a time, if you don't have the chatpad (part of thinks this entire update is all a giant conspiracy to sell more Xbox 360 chatpads). Updates can sometimes take forever to hit your Twitter stream, too. Still, it's pretty, and works the best of the new apps.
Facebook uses the standard Xbox tile UI instead of rolling its interface, like Twitter did. Which is disorienting (and disappointing), since you're browsing through a stream horizontally, one choppily-animated tile at a time. Why is the tile-sliding animation so terrible on a monster console like the Xbox 360? We don't know. Like Twitter, it's basic—focused on Newsfeeds. Your groups are ported over, so you can browse their newsfeeds individually, but you literally have to browse one post at a time, which is agonizing, making you far less inclined to comment on updates.
The interface works much better, and feels way more natural, with photo albums. What's interesting is that, at least in the preview, your friends have to link their Xbox Live and Facebook accounts together themselves in order to show up in the "Xbox Live Friends on Facebook" (and vice versa) pages—you can't manually go in and link Jason Chen's accounts so you'll see them together in your app. That might change though, with the final rollout. (Here's some video of it, from Kotaku.)