Users still aren't going to leave Facebook. Facebook realizes this, which is why they can do this now. Your mother's cousin is on Facebook. She won't be on Diaspora or (X). That said, this is pretty cool for its own reasons. Like waterlesscloud said, it looks like they "want to be the source of all social interactions on the web without having monolithic control."
This is definitely a smart move for Facebook -- be willing to give a little on data portability to position themselves as open.
Facebook has a gigantic ecosystem. Thousands of sites use Facebook Connect to provide social context and are predicated on everyone interesting have a Facebook account. A social network is a very complicated webapp -- probably the most complex many of its users interact with on a daily basis -- and literally hundreds of millions of users don't want to have to re-learn that. Remember that we're talking about here: take, for example, the http://www.readwriteweb.com/archives/facebook_wants_to_be_yo... incident. They have such tremendous inertia that even being able to one-click import your data to another app isn't likely to damage them.
This move is kind of like Jobs saying, "all smartphones have problems. We're not perfect. But we're trying to do the best of anyone." At least for the moment, it's changed the discourse from "wow Facebook supports developers terribly" to "hey, Facebook's not entirely a walled garden!": what Scott Adam's called the "high ground maneuver".