"Give me one other part of history where everybody shows up to the same social space."
There's one mega site "everybody" uses, just like how there used to be one mega network "everybody" watched (NBC) and one mega network "everybody" listened to (NBC again under RCA), etc.
The only thing different with social media is that since people are providing their own shit as content, they end up more closely tied to a particular site because of its content than they were with TV, radio, newspapers, bards, etc. That said, Facebook may be a behemoth but it's certainly not the only behemoth - there's Twitter and Youtube, for example. "everybody" is on Twitter, and "everybody" watches videos on Youtube.