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.