Weren't these people alive in 1994? We had social networks based on email. People augmented them with things like Facebook because Facebook is fundamentally different.
The argument that email is like a giant, open version of Facebook has always seemed silly to me. Perhaps it's something about the name "social network" -- which, like all web-related names, was chosen before the inventors knew exactly what they had built -- that gets people focused on the graph. They look at Facebook and see a graph, and then they look at email and they see an even bigger, implicit graph, and then they proclaim that the future of Facebook is email, only somehow it will be smarter.
Facebook is not about graphs. Facebook works because it's a casual, public meeting place for your friends. If personal email is like your home office, and a blog is like a cross between a diary and a magazine, and Amazon is like the mall, and Google is like the Library of Alexandria, then Facebook is like Cheers -- the corner bar where you and your friends and some friends-of-friends and some folks you know from high school can see each other in public, catch up on gossip, maybe flirt a little, maybe play some pool.
Nobody invites all their high school buddies to hang out in their home office, sit around the desk, and watch them open envelopes. And it's not just because the office is kind of boring and stilted and full of business-related memos and unopened junk mail and stale coffee cups. The real reason is that your office is your personal space. It's full of your private stuff, it's a place that you go to get work done, and it's hard to entertain in -- it's a space designed for working with words, or having very detailed one-on-one conversations, not for public socializing.
Facebook itself may die, because the tides of fashion move on, but the notion of a public meeting space for casual acquaintances will continue. There will be other Facebooks, and they won't be based on email.