This is the same generation that coined the term "side-piece phone", no? I don't think they're living their lives in public; I think they're just getting better at compartmentalizing their identity. Kids don't care if Facebook sees everything they post on Facebook, because they would only use Facebook for the kinds of things everyone should see. Other stuff goes on Snapchat.
"Side-piece" as a concept has existed for a while. I don't think coining that term has any meaning; people have been compartmentalizing their dalliances since the dawn of time
I'm not so sure I'd be so quick to dismiss the differences here. Another example is the notion of having a "finsta" vs their normal Instagram. The finsta is a private Instagram they only show to real friends, while their normal Instagram account is for the world.
This isn't merely "compartmentalizing their dalliances," it's compartmentalization of public vs private (or at least quasi-private) self.