It’s smart for Facebook, because it reinforces their role as facilitator of the community… no doubt many people already go to My Barack Obama, but there’s a non-trivial number of people who want to hang out on Facebook and show their friends what matters to them. (Keep in mind, these people are not all college students, not by a long shot.) By embracing this, the campaign ensures that they’re where their supporters want to be, and aren’t forcing them to come to the campaign’s website in order to engage with the campaign.
If you have an interest in social networking applications, you should read the rest of the post from Rick. I think it is a good example of how to engage in an existing community in a somewhat structured way. Much better effort than the hash they made of their campaign effort in MySpace.