Books decay. Digital files get corrupted. Companies rise and fall. I know I'll end up buying things more than once, just like I did with my Depeche Mode collection - at least three times.
When you are changing the physical media, it's a little more understandable (even though you're still really just giving me a license to the content on it and nothing else). When it's already in digital format, it's no longer acceptable to play that sucker's game, as far as I'm concerned.
If you have a hard drive full of images and it fails, it's all gone (better have a backup!), but if you have a box full of photos and your house floods, some of them may still be around.
Analog fades, digital dies, unless it's constantly resuscitated. When you think about it how many times have you found an old photo that you probably wouldn't have kept if you had the choice? Or some old photos or music at your grandparent's house.
What are my grand kids going to find at my house?
A few years ago a similar statement could have been made about Myspace or friendster a few years before that. Would you be willing to make such a statement about either company today?
Would you have been willing to make such a statement about geocities 5 years ago?
Mostly, I think Facebook has achieved the status of a utility. It is entrenched, with a solid business model. It's drastically different than Myspace and Friendster in that way. They were just fads.
Also, GeoCities was at least partially saved. As we develop more computing infrastructure of all kinds, that kind of thing becomes more an more likely.
With a book or a photo, the storage of information, and it's mechanism for display are one and the same. With all digitally encoded media, one must have the appropriate decoder.
If I keep a journal on wordpress.com, it can't be left to grow old and musty, with no effort. It decays absolutely the instant it is not being maintained, where as if I keep a journal on a notepad, it can take up a tiny amount of physical space in the real world, and lie somewhere unmaintained for centuries without decaying to the point of unusability.
As the speed of information creation and transfer has increased, so too has the longevity of that information decreased. From stone tablets, to papyrus, to SMS messages stored on an SD card - the volume of the world's information is now increasing at an astounding rate but would archaeologists be able to discover it thousands of years from now?