This isn't really unique to Facebook. It's called lock-in and it's the desire of every service provider and software maker; they want you to be held hostage to their network, so that even if you want to leave, you can't without potentially serious ramifications (like losing contact with your grandma, or losing access to your documents).

Facebook has reached a level that had never been reached by any social network before it. Did your grandma use MySpace or LiveJournal? Once something crosses the threshold of youth and tech-savvy and "regular people" or even old people start using it, you have a much less transient userbase. Look at Microsoft -- the young and tech-savvy are happy to use Apple products over MS, but MS is king and will remain king at least until anyone born before 1975 is well into retirement. Once these things reach a certain level, they become generational standards, and then only minimal effort is needed on the part of the vendor.

I know old people that still use AOL. Most old people never would have left AOL in the first place if they'd come up with something sensible for broadband. Perhaps Facebook will come upon a stumbling block that's similarly fatal, but I wouldn't necessarily expect that, at least not within a reasonable timeframe.