Sorry, I don't understand how email could be seen as public, let alone the phone. (Except in as much as they're not secure channels, but neither is facebook.)
I think the poster means that for the average person Facebook is the channel you maintain for people you know publicly (i.e., weak social interactions) to contact you, and email/phone is for your inner circle.
For some of us it's the other way around: email and phone is how people we don't know well contact us, reserving Facebook and Twitter for closer interactions.
For some of us it's the other way around: email and phone is how people we don't know well contact us, reserving Facebook and Twitter for closer interactions.