Privacy and Security
Good points about Facebook's privacy and security issues on their consumer version.
An enterprise system would have to have better controls, administrator controls in addition to user controls, and enterprise level security (let's assume that's better than the consumer version and enterprise FB lives behind the company firewall).
Communications, in some companies, have to pass the Audit test, so many more reporting mechanisms would be needed. As well as archiving messages and communications for potential legal issues. They also have to fix the interface a bit, and even change the appearance to differentiate it from the consumer version.
If FB is just planning on rolling out the same consumer version to the entperprise, well, that's probably a bad plan. Let's see how it goes.