If there is one analogy you should avoid when trying to explain the potential uses of social software for enterprises, it is the Facebook analogy. In my experience, there is no better way to scare an executive or social media skeptic than to use the words "Facebook" and "enterprise" in the same sentence.
Why? Well, because most people who hear the word "Facebook" do not see beyond Facebook (which is not that strange if you think about it). What most of them see is likely a web site where they can connect with friends, tell them what they're up to, and share photos, links and video clips with each other. Some - still a minority I hope - see it as a place where they play Farmville. Social media sceptics (a.k.a. non-Facebook users) see it as a site where people seem to spend a lot of time doing private stuff at work. The typical executive or manager sees it as a productivity drainer.
What is true or not does not matter. What matters is people's perceptions of Facebook.
The point here is that most users don't think about what's under the hood of Facebook and how that technology can be used for other purposes in other contexts, such as for professional networking or knowledge sharing within an enterprise context. It's just too abstract. That's why the "Facebook for the enterprise" expression does more damage than good to Enterprise 2.0.
So stop using the Facebook analogy right away! If you feel a desperate need to make an analogy between an external social networking site and enterprise social networking, then show good judgement by choosing LinkedIn instead of Facebook. In contrast to Facebook, LinkedIn is at least something most people associate with professional use.
(The great folks at NewsGator have used this expression before but now seems to have dropped it. I hope no-one takes this personal, as I'm only playing with words in order to make what I think is an important point.)