Impetus / further example: A friend of mine took the decision to remove himself from social networks for privacy reasons. While I respect his decision and motivation, the loss of comments on our holiday photos from his albums was a real shame.
Further application: 1. Access to historical data/ versioning of your own and others pages (ahem, all sorts privacy issues there). 2. Those popular map-the-people-i-know graphics could be created as videos showing growth over time. 3. Generating a Lulu book from the collected data (I think we all know at least one Facebook addict who would love this).
Of course the irony is that Facebook would likely have ban you for using anything that mined your account at any sort of interval. To avoid suspicion it would need to be "watching" (or mimicking) your regular access and saving the juicy bits rather than logging in for you and crawling.