The parallels between what happened to cities like Chicago, Detroit and New York in the 20th century and what’s happening on the Internet since the introduction of the App Store are striking. Like the great modern American cities, the Web was founded on equal parts opportunism and idealism. Over the years, nerds, students, creeps, outlaws, rebels, moms, fans, church mice, good-time Charlies, middle managers, senior citizens, starlets, presidents and corporate predators all made their home on the Web. In spite of a growing consensus about the dangers of Web vertigo and the importance of curation, there were surprisingly few “walled gardens” online — like the one Facebook purports to (but does not really) represent.

But a kind of virtual redlining is now under way. The Webtropolis is being stratified. Even if, like most people, you still surf the Web on a desktop or laptop, you will have noticed pay walls, invitation-only clubs, subscription programs, privacy settings and other ways of creating tiers of access. All these things make spaces feel “safe” — not only from viruses, instability, unwanted light and sound, unrequested porn, sponsored links and pop-up ads, but also from crude design, wayward and unregistered commenters and the eccentric ­voices and images that make the Web constantly surprising, challenging and enlightening.