Here's my thoughts on the update.
Twitter - Basic, but pretty similar to how the site operates. Can't see myself using it much on the 360, but still nice for anyone who is an addict.
Last.FM - All the functionality of the website is there. Scrobbling is there, you can listen to your personalized stations, random playlists, band playlists, anything of the sort. Can definitely see myself using this a bit. Currently had errors with scrobbling however, so I hope that gets ironed out.
Facebook - Is a bit too basic, but leaving out wall functionality and such like that. However, you CAN comment on others photos unlike what was said above. To comment on one's status you have to do it through the news feed as well and can't do it from their profile itself which is a bbit odd.
Overall, it's a decent update. Improvements can still be made all around, but not a bad start for expanding the 360 into a much more social networking friendly device.