Of course, anyone could write a Python script (or even lash together some shell script involving wget) that would spider your friends' Profile pages and any of your own relevant information (all messages and wall-to-wall threads). Then you would have your own local copy of everything, and would no longer be at their mercy if they suddenly closed up shop.
Hell, you could set it up as a cron job that's done every night. It would look to Facebook almost like the kind of activity I've seen from several users, clicking through all their friends to look at pictures, wall posts, and other crap.
I'm not saying that the current setup is optimal; I'm just saying that backups are certainly not impossible.
Is that kind of thing safe, though? With all probability, their robots.txt and user agreement disallows it. So if they detect your spider, they might shut down your account?
Hell, you could set it up as a cron job that's done every night. It would look to Facebook almost like the kind of activity I've seen from several users, clicking through all their friends to look at pictures, wall posts, and other crap.
I'm not saying that the current setup is optimal; I'm just saying that backups are certainly not impossible.