You're being excessively literalist about this important point.
Either we're living through the digital dark ages, and any and all personal records we make will be toast within 30 years, or we're going to leave a lot of information about ourselves cluttering up the datasphere. Whether it's curated by Facebook or by some other successor entity is beside the point: the question is, what is the net going to be like when more than half the people with a presence on it are dead?
I'm not sure if it is excessively literal, though; if Facebook disappears within 10 or 30 or 50 years, presumably all of the data on Facebook will disappear with it.
Even ancient civilisations kept orders of magnitude more records than have survived now, and they even had the added bonus of being physical — with linguistics the only obstacle to future generations' reading of their content. Our mass of data that seems so permanent is wrapped up in proprietary file formats; it's stored on volatile media; and it's kept around by companies who, in the context of centuries, are flashes in the pan.
How much of the digital information you possessed even ten years ago is still readily accessible? I know in my case there's not a great deal. Now extrapolate that forward 50, 100, 150 years…
How much of the digital information you possessed even ten years ago is still readily accessible?
Most of it. I've been careful.
(Now if you asked about twenty years ago you'd get a different answer; I think I've lost about 75% of the email, but most of the actual written material survives. 1992 is roughly when I got serious about personal data retention.)
