Plot for forthcoming young adult novel The Future of Us: Two teenagers go online in 1996, and discover a wormhole to their future Facebook profiles, thereby allowing them to meet their future selves and learn life lessons.

But then they step on a butterfly and when they return to 2011, the Nazis won World War II and the Winklevii control Facebook!

Sidenote: If my teen self saw my adult self's Facebook profile, I'm pretty sure her response would be, "The boobs don't get bigger than that?" [NYT, Amazon]