Social-media companies quickly learned that users adjust better to "opt-in" features than options that are forced on them. But that's not always a good thing, especially when it comes to facial-recognition software, writes Evgeny Morozov at Slate. Opt-in features can still become basically inescapable. "To assume that a given technology isn't problematic because its users can turn it off seems misguided. Why disregard the possibility that, once enough people opt in to use it, the collective adoption of this technology might dramatically transform the social environment, making nonuse difficult or impossible?"