From what I can tell, given the new Facebook UI, all Facebook needs to do to implement twitter is:
* Streamline directed relationships (fan versus friend) for "follow" functionality
* Streamline status privacy settings (so that changing your status can be equivalent to a tweet)
* Write lite.facebook.com that looks just like twitter/jaiku/etc
In addition, it seems to me that Facebook has the following advantages over twitter:
* Funding: 500mm versus 50mm
* Scale Facebook has many more users and engineers
* Platform/Apps Facebook has a rich platform, twitter doesn't
Thoughts?