Not bad, especially for a system that’s keyboard-less for most users
According to Microsoft spokesman David Dennis, the first-week figures show that at least 2 million Xbox Live users have logged into Facebook, and that half a million Last.fm accounts were created in the first 24 hours of availability. Dennis didn't address how many Xbox Live users have used the service's Twitter feature, except to say that there have been "tweets from nearly every market where we have Xbox Live."
So, based on the data Dennis provided, the Facebook integration with Xbox Live has had the most adoption. And while 2 million people logging into Facebook is far short of the 20 million total Xbox Live users, it is notable that fully one tenth of the service's users have tried the Facebook feature in just the first week.