In Facebook, we get to create our publics. In Twitter, we decide which publics to join. But neither is the public sphere; neither entails publishing to everyone. Yet Facebook is pushing us more and more to publish to everyone and when it does, we lose control of our publics. That, I think, is the line it crossed.
There is a difference between 'the public' and 'publics'. Nissenbaum's new book Privacy in Context is also interesting in this respect.