Maybe, but the thing is...Facebook is pretty awesome.
I know that probably sounds a little juvenile, but that's the attitude that most of its users have. They could care less about having control of their data. The vast majority have no clue that Facebook could give them control of their data, and even if it happened they wouldn't know what to do with it.
"Hey neat, I downloaded my Facebook. I wonder if I'm supposed to play XML files in iTunes or Powerpoint..."
The only thing that millions of people would (not could) potentially do with their Facebook data is: move away from Facebook, given a better alternative. I haven't yet seen a solid argument for why Facebook should open up that data. Maybe there are decent ethical/moral/rhetorical reasons, but none which make business sense for Facebook and the majority of their customers...
Customers will care about controlling their data as soon as someone offers a useful product that requires it. Many are already reluctant to get involved with Facebook because of this.
As it is, MySpace users who want to switch to Facebook do it anyway, leaving behind a GhostSpace where they log into to check messages maybe once every 2-3 months. People are going to get sick of that crap, and will switch to a service that promises they'll never have to worry about it again.
If it was possible to export that data there would be apps to help people to process it further. No need to stumble around in XML piles. Your "makes no business sense" argument is not even Web 1.0 ;-) It's the ages old argument that was brought forward wrt Office file formats, pre TCP/IP network protocols and an endless list of other lock-in strategies. Even Microsoft has shed that retro attitude to some degree exactly because it does not make business sense in the long run. You need third parties to build on your stuff and that means you must set the data free.
But people only care when 3rd-party tools exist that they can't use. Right now, the ivory tower is okay, because the rest of the landscape is flat and barren.
I think people do care a lot about moving data between applications. There is nothing that comes up more often when end users need help. Today I was asked to move emails out of Mac Mail. Last month it was watch lists from a trading site to an Access database. Data silos are a huge problem and a productivity killer.
Others have said this in different ways before but it bears repeating.