Privacy didn't need protecting in history before bugging. If a founding father wanted to talk to someone privately they just went stood apart from everybody else.
50 years ago, standing apart no longer provided privacy because of long range microphones etc, but those were targeted attacks on diplomats etc.
Nowadays, you can probably do a nice undergrad project to recreate conversations from lip reading streetview video clips.
Privacy was defacto available hundreds of years ago, but is technically impossible now. (Imagine the kinds of body checks that will be introduced before you enter a US SCIF post the chess cheating scandal!)
The Constitution is not an enumeration of rights that the people have. It even says so itself, in the ninth amendment. So there doesn't need to be text which says you have a right to privacy: you have that right even without it being listed.
But more importantly, there are huge differences between internet anonymity and the older comceptualization of a right to privacy, which has mostly to do with shielding conversations between people who know each other's identities (real names and addresses) from prying government eyes. Shielding such conversations from prying government eyes is not incompatible with preventing teenagers from using social media.